Avoid false positive generation oops in navigate and march

Commit e3cf1e32 (v4.3.27) created make_stale_if_command_arg() to
permit catching more potential yields on input.  Unfortunately, the
implementation of navigate and march sub-commands 'r', 'l' and 's'
breaks it.

do_unit_move() reads units into a unit list at the beginning and at
each stop.  It writes them back when they move or sweep.  If a unit
changed in the file in between, the changes would get wiped out.
Therefore, do_unit_move() must not yield between stops.

do_unit_move() parses sub-commands into player->argp[], then supplies
defaults for missing arguments, so that code using them (radar(),
do_look(), sona(), mine(), landmine()) won't prompt for missing
arguments.  Unclean and brittle.  See also commit 28cc236e and commit
45106ab9.

Unfortunately, make_stale_if_command_arg() doesn't recognize the
difference between these defaulted arguments and parsed arguments, so
it makes objects stale, even though the defaulted arguments can't be
missing.  If a move or sweep follows, it triggers a false positive
generation oops.

To fix, test "points into argument buffer" (only true for parsed
arguments) instead of "is in player->argp[]".  Requires making the
argument buffer accessible: new struct player member argbuf[].  Use it
for parsing commands, in command(), execute(), do_unit_move().  Don't
use it in emp_config(), player_login(), move_ground(), because these
parse something else.

include/player.h

@@ -68,6 +68,7 @@ struct player {
     struct cmndstr *command;	/* currently executing command */
     struct iop *iop;
     char combuf[1024];		/* command input buffer, UTF-8 */
+    char argbuf[1024];		/* argument buffer, ASCII */
     char *argp[128];		/* arguments, ASCII, valid if command */
     char *condarg;		/* conditional, ASCII, valid if command */
     char *comtail[128];		/* start of args in combuf[] */