Presence of any VM_PROT bits in the permission argument on x86 implies
that the entry is readable and valid.

Reported by:	markj
Submitted by:	alc
Tested by:	pho (previous version), markj
MFC after:	3 days
Revert unrelated changes accidentally committed in r271192.
Correct a comment
For targets that have a signed zero, lgamma_r(-0, &signgamp) should
set signgamp = -1.

Submitted by:	enh at google dot com (e_lgamma[f]_r.c)