Remove $FreeBSD$: two-line .h pattern

Remove /^\s*\*\n \*\s+\$FreeBSD\$$\n/

localedef: Fix bootstrapping on Ubuntu 16.04

Glibc's stdlib.h defines various prototypes for GNU extensions that take
a locale_t. Newer versions use locale_t directly and include an internal
bits/types/locale_t.h in order to get its definition, but older versions
include xlocale.h for that, for which our bootstrap version is empty.
Moreover it expects to use the glibc-specific __locale_t type. Thus,
provide dummy definitions of both types in order to ensure the
prototypes don't give any errors, and guard against the header being
inadvertently included between the bootstrapping namespace.h and
un-namespace.h, where locale_t is #define'd.

This header is not used when bootstrapping on FreeBSD and exists solely
to stub out glibc's, so this should have no impact on FreeBSD hosts.

Reviewed by: arichardson, emaste (comment only)
Differential Revision: https://reviews.freebsd.org/D28317

Allow bootstrapping localdef on non-FreeBSD systems

The current localedef simply assumes that the locale headers on build system
are compatible with those on the target system which is not necessarily true.
It generally works on FreeBSD (as long as we don't change the locale headers),
but Linux and macOS provide completely different locale headers.

This change adds new bootstrap headers that namespace certain xlocale
structures defined or used by in the headers that localdef needs.
This is required since system headers *must* be able to include the "real"
locale headers for printf(), etc., but we also want to access the target
systems's internal locale structures.

Reviewed By: yuripv, brooks
Differential Revision: https://reviews.freebsd.org/D25229