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ypldap: Fix mismatch in array bounds for ldapclient().

Reviewed by: emaste
Reported by: GCC -Warray-parameter
Differential Revision: https://reviews.freebsd.org/D36807

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

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

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

Replace __dead with __dead2.

FreeBSD lacks __dead.

Replace HOST_NAME_MAX with MAXHOSTNAMELEN.

FreeBSD lacks HOST_NAME_MAX.

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.