subs: Simplify MOB_ACCESS mobility update

subs: Simplify MOB_ACCESS mobility update

The do_upd_checking recursion guard is superfluous: do_mob_sect()
doesn't call anything.  Has been that way since MOB_ACCESS was added
in Empire 3.

Inline the remaining code of sct_do_upd_mob(), shp_do_upd_mob(),
pln_do_upd_mob(), lnd_do_upd_mob() in their only callers
sct_postread(), shp_postread(), pln_postread(), lnd_postread().
Rename do_mob_sect(), do_mob_ship(), do_mob_plane(), do_mob_land() to
mob_sect, mob_ship(), mob_plane(), mob_land() and give them external
linkage.

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