MFC r309392, r309393

r309392:
String terminators are called NUL, not NULL, also the variable
mentioned in the comment is p and not u.

Obtained from: OpenBSD (r1.34)

r309393:
Use memset(3) instead of bzero(3).

Obtained from: OpenBSD (r1.12, r1.20, r1.18 and r1.37)

Copy head@r302406 to stable/11 as part of the 11.0-RELEASE cycle.
Prune svn:mergeinfo from the new branch, as nothing has been merged
here.

Additional commits post-branch will follow.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Convert ypldap_addr list to a tailq(queue(3)).

Obtained from: OpenBSD r1.11, r1.17 and r1.36

EAGAIN handling for imsg_read.

Approved by: bapt (mentor)
Obtained from: OpenBSD
Differential Revision: https://reviews.freebsd.org/D4547

Add include of sys/param.h for MAXHOSTNAMELEN

Remove reference to EAI_NODATA.

It does not exist on FreeBSD.

Import ypldap from OpenBSD.

ypldap -- Intended to be a drop-in replacement for ypserv, gluing in a
LDAP directory and thus providing support for users and groups stored in
LDAP for the get{pw,gr}ent family of functions.