* perthread.h (vfork_save): Add ctty, sid, pgid, exitval fields.

* perthread.h (vfork_save): Add ctty, sid, pgid, exitval fields.
(vfork_save::restore_pid): New method.
(vfork_save::restore_exit): New method.
* fork.cc (vfork): Save ctty, sid, pgid and restore them when returning to
"parent".  Use exitval field if exiting but never created a new process.
* syscalls.cc (setsid): Detect when in "vfork" and force an actual fork so that
pid will be allocated (UGLY!).
(getsid): New function.
* dcrt0.cc (do_exit): Use vfork_save::restore_exit method for returning from a
vfork.
* spawn.cc (spawnve): Use vfork_save::{restore_pid,restore_exit} methods for
returning from vfork.
* cygwin.din: Export getsid.
* include/cygwin/version.h: Bump api minor number.
* malloc.cc: #ifdef sYSTRIm for when MORECORE_CANNOT_TRIM is true.