Sync our copy of the timezone library with IANA release tzcode2020c.

 Sync our copy of the timezone library with IANA release tzcode2020c.
 
 This changes zic's default output format from "-b fat" to "-b slim".
 We were already using "slim" in v13/HEAD, so those branches drop
 the explicit -b switch in the Makefiles.  Instead, add an explicit
 "-b fat" in v12 and before, so that we don't change the output file
 format in those branches.  (This is perhaps excessively conservative,
 but we decided not to do so in a12079109, and I'll stick with that.)
 
 Other non-cosmetic changes are to drop support for zic's long-obsolete
 "-y" switch, and to ensure that strftime() does not change errno
 unless it fails.
 
 As usual with tzcode changes, back-patch to all supported branches.