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

- 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.

Reviewed by:	imp
Differential Revision:	https://reviews.freebsd.org/D15831