lib/timeutils: add common ISO timestamp masks

lib/timeutils: add common ISO timestamp masks

* Start the ISO format flags at bit 0 instead of bit 1.

* Remove unnecessary _8601 from ISO format flag names to
  avoid line wrapping and to ease readability.

* ISO timestamps have date-time-timzone in common, so move
  the TIMEZONE flag to bit 2 causing all timestamp masks
  to have the first three bits set and the last four bits
  as timestamp 'options'.

* Change the 'SPACE' flag to a 'T' flag, because it makes
  the code and comments more concise.

* Add common ISO timestamp masks.

* Implement the ISO timestamp masks in all applicable code
  using the strxxx_iso() functions.

Signed-off-by: J William Piggott <elseifthen@gmx.com>