Remove $FreeBSD$: one-line sh pattern

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

ident: replace sbuf(9) with open_memstream(3)

This change makes ident only dependant on libc functions

This makes our ident(1) more portable, also the fact that we only
depend on libc which is maintained with excellent backward compatibility
means that if one day ident is removed from base, someone using FreeBSD
22 will be able to fetch ident from FreeBSD 14 to run ident against
FreeBSD 1.0 binary

MFC After: 1 week

Add HAS_TESTS to all Makefiles that are currently using the
`SUBDIR.${MK_TESTS}+= tests` idiom.

This is a follow up to r321912.

Convert traditional ${MK_TESTS} conditional idiom for including test
directories to SUBDIR.${MK_TESTS} idiom

This is being done to pave the way for future work (and homogenity) in
^/projects/make-check-sandbox .

No functional change intended.

MFC after: 1 weeks

Replace GNU RCS ident with a BSD license ident

Rationale: ident(1) is useful out of RCS, lot of scripts are using ident(1) and
failing when base is built WITHOUT_RCS.

This version is:
- fully compatible with RCS 5.7 ident.
- fully compatible with RCS 5.9 ident.
- passes all ident test from GNU RCS 5.9 test suite

This version has support for: svn extension for the Keyword id (double colon and
# before last $)

Différences with GNU RCS ident:
- no long options as found in GNU RCS 5.9 (but not commented there).
- '-V' reports nothing but has been added for compatibility.

Differential Revision: https://reviews.freebsd.org/D3200
Reviewed by: pfg