Somewhat modernize the SysV shm code:

• Use real locking, replace Giant with global sx protecting the subsystem. Since the subsystem' lock is no longer dropped during the sleepsk, remove not needed SHMSEG_WANTED segment flag, and revert r278963.
• To do proper code simplification possible after the change of the lock, restructure several functions into _locked body and originally-named wrapper which calls into _locked variant. This allows to eliminate the 'goto done2' spread over the code.
• Merge shm_find_segment_by_shmid() and shm_find_segment_by_shmidx().
• Consistently change all function prototypes to ANSI C.

Reviewed by: mjg (who has earlier version of the similar patch to

	 introduce real locking)

Tested by: pho
Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks