ypldap(8): Fix grammar in ypldap.c

Event: Advanced UNIX Programming Course (Fall’23) at NTHU.
Pull Request: https://github.com/freebsd/freebsd-src/pull/991

Trim various $FreeBSD$

Approved by: markj (cddl/contrib changes)
Reviewed by: imp, emaste
Differential Revision: https://reviews.freebsd.org/D41961

yp*: fix -fno-common build

This is mostly two problems spread out far and wide:
- ypldap_process should be declared properly
- debug is defined differently in many programs

For the latter, just extern it and define it everywhere that actually needs
it. This mostly works out nicely for ^/libexec/ypxfr, which can remove the
assignment at the beginning of main in favor of defining it properly.

-fno-common will become the default in GCC10/LLVM11.

MFC after: 3 days

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

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

When a group contains a non-existent user, make the warning
message more helpful by mentioning the group name.

Obtained from: OpenBSD (cvs 1.19)

EAGAIN handling for imsg_read.

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

Replace __dead with __dead2.

FreeBSD lacks __dead.

Add include of sys/param.h for MAXHOSTNAMELEN

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.