Remove $FreeBSD$: two-line .h pattern

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

spdx: The BSD-2-Clause-FreeBSD identifier is obsolete, drop -FreeBSD

The SPDX folks have obsoleted the BSD-2-Clause-FreeBSD identifier. Catch
up to that fact and revert to their recommended match of BSD-2-Clause.

Discussed with: pfg
MFC After: 3 days
Sponsored by: Netflix

rtld/tests: Avoid function name conflict with libc opendir()

This prevents these tests from being compiled with ASAN since the asan
interceptors also define opendir() but matching the libc function.

Reviewed By: oshogbo, kib, markj
Differential Revision: https://reviews.freebsd.org/D31038

rtld: introduce PRELOAD_FDS

The new PRELOAD_FDS variable accepts a list of file descriptors
that should be loaded into the process.

This may be used to optimize a loading process - in the case when
we already have a file descriptor to the library; we don't have
to look into multiple PATH to find it.

It may also be used in capability mode to load a single additional
library without the need to open a directory that contains it.

The last use of this functionality t may be a race-free method
of loading libraries.

Reviewed by: kib, markj
Differential Revision: https://reviews.freebsd.org/D29334