Clean up errno reporting and fix some errno-reporting bugs.

* callproc.c (Fcall_process):
* fileio.c (Fcopy_file, Finsert_file_contents, Fwrite_region):
* process.c (create_process, Fmake_network_process):
* unexaix.c (report_error):
* unexcoff.c (report_error):
Be more careful about reporting the errno of failed operations.
The code previously reported the wrong errno sometimes.
Also, prefer report_file_errno to setting errno + report_file_error.
(Fcall_process): Look at openp return value rather than at path,
as that's a bit faster and clearer when there's a numeric predicate.
* fileio.c (report_file_errno): New function, with most of the
old contents of report_file_error.
(report_file_error): Use it.
(Ffile_exists_p, Ffile_accessible_directory_p):
Set errno to 0 when it is junk.
* fileio.c (Faccess_file):
* image.c (x_create_bitmap_from_file):
Use faccessat rather than opening the file, to avoid the hassle of
having a file descriptor open.
* lisp.h (report_file_errno): New decl.
* lread.c (Flocate_file_internal): File descriptor 0 is valid, too.

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