Assume POSIX 1003.1-1988 or later for errno.h.

Assume POSIX 1003.1-1988 or later for errno.h.

* lib-src/movemail.c (main): Assume EAGAIN and EBUSY.
* src/dired.c (directory_files_internal, file_name_completion):
Assume EAGAIN and EINTR are defined.
* src/fileio.c (Fcopy_file): Assume EISDIR is defined.
* src/gmalloc.c (ENOMEM, EINVAL): Assume they're defined.
* src/gnutls.c (emacs_gnutls_write): Assume EAGAIN is defined.
* src/lread.c (readbyte_from_file): Assume EINTR is defined.
* src/process.c (wait_reading_process_output, send_process) [subprocesses]:
Assume EIO and EAGAIN are defined.
* src/unexcoff.c (write_segment): Assume EFAULT is defined.

Fixes: debbugs:12968