usr.bin: Remove ancient SCCS tags.

Remove ancient SCCS tags from the tree, automated scripting, with two
minor fixup to keep things compiling. All the common forms in the tree
were removed with a perl script.

Sponsored by: Netflix

flex: Regen bootstrap files

This also partially reverts r326025 (8a16b7a18f5d). I do not see any
point of adding SPDX tag in generated file.

MFC after: 3 days
Submitted by: Dan McGregor <dan.mcgregor@usask.ca> (initial version)
Differential Revision: https://reviews.freebsd.org/D28596

Fix broken SPDX comment in usr.bin/lex/initparse.c

Approved By: jhb (mentor)

General further adoption of SPDX licensing ID tags.

Mainly focus on files that use BSD 3-Clause license.

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.

Special thanks to Wind River for providing access to "The Duke of
Highlander" tool: an older (2014) run over FreeBSD tree was useful as a
starting point.

Use NULL instead of 0 for pointers.

realloc will return NULL in case it cannot allocate memory.

MFC after: 2 weeks.

Connect flex 2.5.37 to the build and bump __FreeBSD_version.