retreat: Don't charge mobility for retreating in direction 'h'

retreat: Don't charge mobility for retreating in direction 'h'

Obscure feature: 'h' in a retreat path stops the current retreat.  The
code treats that as entering the current sector again, thus charges
mobility for staying put.  It also reports "could not retreat to" for
a ship or land unit that can retreat out of, but could not retreat
into its current sector, e.g. a ship in an unfriendly harbor.

Fix by cleaning up the tortuous control flow.

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