This changes the API slightly, in a hopefully-innocuous way.
Without this change the code had undefined behavior when a
caller specified neither PDS_LEADING_YEAR nor PDS_TRAILING_YEAR.
Problem reported by Pádraig Brady in:
https://lists.gnu.org/r/bug-gnulib/2017-11/msg00048.html
* NEWS: Mention this.
* lib/posixtm.c (posix_time_parse): Treat the absence of
PDS_TRAILING_YEAR as if PDS_LEADING_YEAR were present.
* lib/posixtm.h (PDS_LEADING_YEAR): Remove (actually, leave it
present, but define it as zero, for compatibility with existing
source code). All other PDS_* values moved up.
* tests/test-posixtm.c (LY): New macro.
(T): Use it. Do not expect a particular numeric encoding
for PDS_CENTURY etc.