* Makefile.in: Remove cygserver stuff.

* Makefile.in: Remove cygserver stuff.
* acconfig.h: Add USE_CYGSERVER define.
* config.h.in: Regenerate.
* configure.in: Add --enable-server setting.
* configure: Regenerate.
* fhandler_tty.cc (fhandler_tty_slave::open): Conditionalize compilation of
cygserver stuff.
* fork.cc (fork_child): Ditto.
* shm.cc: Ditto.
* tty.cc (tty::common_init): Ditto.
* dcrt0.cc: Use bool rather than BOOL for CYGWIN environment variable
definitions.
* environ.cc: Ditto.
* ntea.cc: Ditto.
* security.cc: Ditto.
* security.h: Ditto.
* syscalls.cc (check_posix_perm): Remove externs that were already declared in
a header.
* winsup.h: Ditto.  Declare _MT_SAFE here.  Delete it someday since cygwin
should always be _MT_SAFE.