MFC r241855,r241859:

MFC r241855,r241859:
	Update the kill(2) and killpg(2) man pages to the modern permission
	checks. Also indicate killpg(2) is POSIX compliant.

	Correct the killpg(2) return values:

	Return EPERM if processes were found but they
	were unable to be signaled.

	Return the first error from p_cansignal if no signal was successful.

Discussed with:	jilles
Approved by:	cperciva (implicit)

git-svn-id: svn://svn.freebsd.org/base/stable/9@242283 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f