Remove $FreeBSD$: one-line .c pattern

Remove /^[\s*]*__FBSDID\("\$FreeBSD\$"\);?\s*\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spdx: The BSD-2-Clause-FreeBSD identifier is obsolete, drop -FreeBSD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

various: general adoption of SPDX licensing ID tags.

Mainly focus on files that use BSD 2-Clause license, however the tool I
was using misidentified many licenses so this was mostly a manual - error
prone - task.

The Software Package Data Exchange (SPDX) group provides a specification
to make it easier for automated tools to detect and summarize well known
opensource licenses. We are gradually adopting the specification, noting
that the tags are considered only advisory and do not, in any way,
superceed or replace the license texts.

No functional change intended.

Remove utxrm(8). This command has been superseded by 'utx rm'.

Replace utxrm(8) by utx(8).

At first, I added a utility called utxrm(8) to remove stale entries from
the user accounting database. It seems there are cases in which we need
to perform different operations on the database as well. Simply rename
utxrm(8) to utx(8) and place the old code under the "rm" command.

In addition to "rm", this tool supports "boot" and "shutdown", which are
going to be used by an rc-script which I am going to commit separately.