* lib/nstrftime.c (width_add, width_add1, width_cpy):
New macros, which generalize ‘add’, ‘add1’, ‘cpy’ by adding
a new WIDTH parameter.
(add, add1, cpy): Use these macros.
(width_add): Do not treat digits == 0 as a special case,
do not pad if padding is ‘-’, and do not use a negative width.
(__strftime_internal): Redo formatting of nanoseconds and numeric
timezones to avoid buffer misuse in unusual cases, and so that
widths make more sense. Add support for widths greater than 9 to
the %N format; they are zero filled on the right.
* tests/test-nstrftime.c (posixtm_test): Add a %12N test.