* errno.cc (__xpg_strerror_r): New function.
(strerror_r): Update to copy newlib's fixes.
(strerror): Set errno on failure.
(_sys_errlist): Cause EINVAL failure for reserved values.
* cygwin.din: Export new function.
* include/cygwin/version.h (CYGWIN_VERSION_API_MINOR): Combine
this into minor 236.