tests: head-c: avoid spurious failure with a 32-bit size_t

* tests/misc/head-c.sh: Don't try to elide 1 exabytes, since on
32-bit systems, that number is not representable as a size_t.
This command would fail on 32-bit systems, where SIZE_MAX < 1E:
  head --bytes=-E < /dev/null
Instead of "E", use $SSIZE_MAX.
For discussion, see http://bugs.gnu.org/13530
od: -wN, N>64K, avoid misbehavior on systems with 32-bit size_t

* src/od.c (PRINT_FIELDS): Declare "i" to be of type uintmax_t, so that
the numerator in the expression for "next_pad" does not overflow.
(print_named_ascii): Likewise.
(print_ascii): Likewise.
Bug introduced via commit v6.12-42-g20c0b87.
* tests/misc/od.pl: Exercise each of the three affected code paths.
* NEWS (Bug fixes): Mention it.
Reported by Rich Burridge.
doc: mention 'UTC' in date --utc help

src/date.c (usage): Make -u apparent in searches for UTC.