* fork.cc (fork): Use above defines instead of numerical constants.
* shared.cc (sec_user): Ditto.
* shared.h (class pinfo): Ditto.
* syscall.cc (seteuid): Ditto.
* spawn.cc (_spawnve): Ditto. Eliminate conditional.
(spawn_guts): Set child->uid = USHRT_MAX when user context will be
changed in child process.
* uinfo.cc (uinfo_init): Check for myself->uid instead of myself->psid
to avoid reloading of /etc/passwd on process startup if ntsec is off.
Use above defines instead of numerical constants.
* security.cc: Move define for MAX_SID_LEN to winsup.h.
