posix_spawn_file_actions_adddup2(3): Document difference with dup2().

The ability to clear a file descriptor's close-on-exec flag via
posix_spawn_file_actions_adddup2() is in fact proposed in Austin Group issue
#411.

MFC after:	1 week
Fix two typos

Reviewed by:	jilles
Retire write-only PCB_GS32BIT pcb flag on amd64.