errno: Port to Minix.

errno: Port to Minix.

* m4/errno_h.m4 (gl_HEADER_ERRNO_H): Test also whether ENETRESET and
ECONNABORTED are defined.
* lib/errno.in.h (ENETRESET, GNULIB_defined_ENETRESET, ECONNABORTED,
GNULIB_defined_ECONNABORTED): New macros.
* lib/strerror-override.h (strerror_override): Test also
GNULIB_defined_ENETRESET, GNULIB_defined_ECONNABORTED.
* lib/strerror-override.c (strerror_override): Handle also ENETRESET,
ECONNABORTED.
* doc/posix-headers/errno.texi: Mention the Minix problem.