Bring malloc(9) and <sys/malloc.h> into sync for M_* entries, such

Bring malloc(9) and <sys/malloc.h> into sync for M_* entries, such
that the former's text comes from the latter's comments.  Rationalize
capitalization, whitespace, and plural-vs-singular.  Mark things for
for automation in the future.

Prompted by loss of sync from the addition of M_IFGROUP and M_PF
Previously worked up in discussion with schwarze@ and jmc@

ok deraadt@, miod@, jmc@