sail: Remove option SAIL

SAIL has issues:

* Sail orders are executed at the update.  Crafty players can use them
  to get around the update window.

* The route is fixed at command time.  You can't let the update find
  the best route, like it does for distribution.

* The info pages documenting it amount to almost 100 non-blank lines
  formatted.  They claim you can follow friendly ships.  This is
  wrong.  They also show incorrect follow syntax.  Unlikely to be the
  only errors.

* Few players use it.  Makes it a nice hidey-hole for bugs.  Here are
  two nice ones:

  - If follow's second argument is negative, the code attempts to
    follow an uninitialized ship.  Could well be a remote hole.

  - If ship #1 follows #2 follows #3 follows #2, the update goes into
    an infinite loop.

* It's more than 500 lines of rather crufty code nobody wants to
  touch.  Thanks to a big effort in Empire 2, it shares some code with
  the navigation command.  It still duplicates other navigation code.
  The sharing complicates fixing the bugs demonstrated by
  navi-march-test.

Reviewing, fixing and testing this mess isn't worth the opportunity
cost.  Remove it instead.  Drop commands follow, mquota, sail and
unsail.  Drop ship selectors mquota, path, follow.

struct shpstr shrinks some more, on my system from 160 to 120 bytes.

Signed-off-by: Markus Armbruster <armbru@pond.sub.org>

info/mission.t

@@ -31,8 +31,6 @@ That's the price you pay for having automatic defenses.
 .s1
 .L "IMPORTANT NOTES"
 .s1
-Missions can occur during updates.  (Interdicting ships using "sail").
-.s1
 Artillery units less than 40% efficient will not fire.
 .s1
 Planes flying missions other than air defense will be intercepted