malloc(9): update names and hardlinks

Give all documented functions a .Nm entry in the man page, following the
order they are listed in the synopsis. Create MLINKs for each of the
functions as well.

While here, add a missing include directive to the synopsis, and appease
mandoc by wrapping a long line.

Reviewed by: markj, imp (previous version), jhb (previous version)
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D36940

(cherry picked from commit 30c6a66864821d3b47e25612c0d2e727ec36154e)