(get_assembly_point): Close loophole that allowed players to find all

unowned sectors.  The assembly point must now be owned or allied, or
have an owned or allied ship.

Info pages didn't match the old rule.  Fix them to match the new rule.

info/Commands/bomb.t

@@ -31,7 +31,8 @@ Only missiles in hardened silos can avoid damage from a nuclear mission.
 .s1
 <SECT> represents an assembly point, where all of the planes in the
 mission meet before proceeding on to the target sector.
-The assembly point must be owned by you, and must not be more than
+The assembly point must be owned by you or an ally, or you or an ally
+must have a ship there.  It must not be more than
 four sectors away from any of the planes selected for the mission.
 .s1
 Route is a normal empire path specification. You may also give a 

info/Commands/drop.t

@@ -17,7 +17,8 @@ To be selected, escorts must have fuel and mobility.
 .s1
 <SECT> represents an assembly point, where all of the planes in the
 mission meet before proceeding on to the target sector.
-The assembly point must be owned by you, and must not be more than
+The assembly point must be owned by you or an ally, or you or an ally
+must have a ship there.  It must not be more than
 four sectors away from any of the planes selected for the mission.
 .s1
 <COMMODITY> is the name of a commodity which is to be carried along

info/Commands/fly.t

@@ -16,7 +16,8 @@ To be selected, escorts must have fuel and mobility.
 .s1
 <SECT> represents an assembly point, where all of the planes in the
 mission meet before proceeding on to the target sector.
-The assembly point must be owned by you, and must not be more than
+The assembly point must be owned by you or an ally, or you or an ally
+must have a ship there.  It must not be more than
 four sectors away from any of the planes selected for the mission.
 .s1
 <COMMODITY> is the name of a commodity which is to be carried along

info/Commands/paradrop.t

@@ -19,7 +19,8 @@ To be selected, escorts must have fuel and mobility.
 .s1
 <SECT> represents an assembly point, where all of the planes in the
 mission meet before proceeding on to the target sector.
-The assembly point must be owned by you, and must not be more than
+The assembly point must be owned by you or an ally, or you or an ally
+must have a ship there.  It must not be more than
 four sectors away from any of the planes selected for the mission.
 .s1
 Route is a normal empire path specification. You may also give a 

info/Commands/recon.t

@@ -18,7 +18,8 @@ To be selected, escorts must have fuel and mobility.
 .s1
 <SECT> represents an assembly point, where all of the planes in the
 mission meet before proceeding on to the target sector.
-The assembly point must be owned by you, and must not be more than
+The assembly point must be owned by you or an ally, or you or an ally
+must have a ship there.  It must not be more than
 four sectors away from any of the planes selected for the mission.
 .s1
 Route is a normal empire path specification. You may also give a 

info/Commands/sweep.t

@@ -17,7 +17,8 @@ To be selected, escorts must have fuel and mobility.
 .s1
 <SECT> represents an assembly point, where all of the planes in the
 mission meet before proceeding on to the target sector.
-The assembly point must be owned by you, and must not be more than
+The assembly point must be owned by you or an ally, or you or an ally
+must have a ship there.  It must not be more than
 four sectors away from any of the planes selected for the mission.
 .s1
 Route is a normal empire path specification. You may also give a 

src/lib/subs/plnsub.c

@@ -64,19 +64,33 @@ get_assembly_point(char *input, struct sctstr *ap_sect, char *buf)
 {
     char *p;
     coord x, y;
+    struct nstr_item ni;
+    struct shpstr ship;
 
     p = getstarg(input, "assembly point? ", buf);
     if (!p || *p == 0)
 	return NULL;
     if (!sarg_xy(p, &x, &y) || !getsect(x, y, ap_sect))
 	return NULL;
-    if (ap_sect->sct_own && ap_sect->sct_own != player->cnum &&
+
-	getrel(getnatp(ap_sect->sct_own), player->cnum) != ALLIED) {
+    /* over own or allied sector is fine */
+    if (ap_sect->sct_own == player->cnum
+	|| getrel(getnatp(ap_sect->sct_own), player->cnum) == ALLIED)
+	return ap_sect;
+
+    /* over own or allied ship is fine */
+    snxtitem_xy(&ni, EF_SHIP, x, y);
+    while (nxtitem(&ni, &ship)) {
+	if (ship.shp_effic < SHIP_MINEFF || ship.shp_own == 0)
+	    continue;
+	if (ship.shp_own == player->cnum
+	    || getrel(getnatp(ship.shp_own), player->cnum) == ALLIED)
+	    return ap_sect;
+    }
+
     pr("Assembly point not owned by you or an ally!\n");
     return NULL;
 }
-    return ap_sect;
-}
 
 int
 pln_onewaymission(struct sctstr *target, int *shipno, int *flagp)