AGOS: Slightly clean up ArchiveMan code.

This removes the (always true) _fallBack flag which determined whether
SearchMan should be searched.
AGOS: Get rid of ArchiveMan in favor of global SearchMan.

This in fact slightly changes the priority order of added archives. Formerly,
all archives in SearchMan were preferred to the customly added ones in
ArchiveMan. All standard paths (i.e. path and extrapath) will be still be
searched before the custom ones (which are all priority 0 right now) but system
specific paths will be searched after (due to their priority being -1). Since
system specific paths shouldn't contain any game data files this should
hopefully be harmless.

This wasn't tested for games with CAB archives.
AGOS: Use Common::File instead of SearchMan.

This makes sure the hack from Common::File for filenames with a trailing dot
is used as expected.
Merge pull request #422 from lordhoto/agos-archive

AGOS: Archive code cleanup