Remove $FreeBSD$: one-line sh pattern

Remove /^\s*#[#!]?\s*\$FreeBSD\$.*$\n/

pkgbase: Put yellow pages programs to its own package

YP is less and less used, split them to users have the choice to not
install them.

MFC after: 2 weeks
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D33441

Only set WARNS if not defined

This would allow interested parties to do experimental runs with an
environment set appropriately to raise all the warnings throughout the
build; e.g. env WARNS=6 NO_WERROR=yes buildworld.

Not currently touching the numerous instances in ^/tools.

MFC after: 1 week

rename in-tree libevent v1 to libevent1

r316063 installed pf's embedded libevent as a private lib, with headers
in /usr/include/private/event. Unfortunately we also have a copy of
libevent v2 included in ntp, which needed to be updated for compatibility
with OpenSSL 1.1.

As unadorned 'libevent' generally refers to libevent v2, be explicit that
this one is libevent v1.

Reviewed by: vangyzen (earlier)
Differential Revision: https://reviews.freebsd.org/D17275

Use SRCTOP-relative paths to other directories instead of .CURDIR-relative ones

This simplifies pathing in make/displayed output.

MFC after: 1 week
Sponsored by: Dell EMC Isilon

Convert Makefile to FreeBSD style.

Add libopenbsd to link line for imsg.

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.