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