strptime: add support for %t and %n

Posix strptime() requires support for %t and %n, which were added
to the illumos port.  Curiously we were skipping white spaces by
default in most other cases making %t meaningless.

We now skip spaces in the case of the %e specifier as strftime(3)
explicitly adds a space for the single digit case.

Reference:
http://pubs.opengroup.org/onlinepubs/009695399/functions/strptime.html

Obtained from:	Illumos (Rev. a11c1571b6942161b0186d0588609448066892c2)
MFC after:	3 weeks