Clean up the vcs ID strings in libc's gen/ directory.

[jhb]

Summary

• Move CSRG IDs into __SCCSID().
• When a file has been copied, consistently use 'From: <tag>' for strings referencing the version of the source file copied from in the license block comment.
• Some of the 'From:' tags were using $FreeBSD$ that was being expanded on each checkout. Fix those to hardcode the FreeBSD tag from the file that was copied at the time of the copy.
• When multiple strings are present list them in "chronological" order, so CSRG (SCCSID) before FreeBSD (FBSDID). If a file came from OtherBSD and contains a CSRG ID from the OtherBSD file, use the order CSRG -> OtherBSD -> FreeBSD.

There are some files that have $NetBSD$ and $OpenBSD$ tags at the start
of the file before the license statement. I have left those as-is, but
they should perhaps be changed to use RCSID? NetBSD stores the tag
both in a comment at the top and in a RCSID()-type string. OpenBSD
used to store both but seems to have removed the __RCSID()-type thing,
so perhaps it is best to leave them as is?

Test Plan

• buildworld plus some cursory examination of .comment contents

[imp]

thanks for doing this largely thankless task.
#ifdef link -= 1000 because of it.