Remove $FreeBSD$: one-line sh pattern

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

Move libmap.conf to libexec/rtld-elf/

This leverages CONFS to handle the config file install.

Approved by: re (gjb), will (mentor)
Differential Revision: https://reviews.freebsd.org/D17161

Include /usr/local/etc/libmap.d/ by default.

PR: 180568
Reviewed by: bapt
Obtained from: kib
MFC after: 3 days