Add functions to open a file without quitting.

Add functions to open a file without quitting.

In some situations, e.g. when the Lisp machinery isn't available, we
can't quit.  Don't check the quit flags in such situations, in case
they contain garbage.

* src/sysdep.c (emacs_open_noquit, emacs_openat_noquit): New variants
of 'emacs_open' and 'emacs_openat' that don't check the quit flags.

* src/emacs.c (main, Fdaemon_initialized):
* src/pdumper.c (pdumper_load):
* src/w32term.c (w32_initialize):
* src/buffer.c (mmap_init):
* src/callproc.c (emacs_spawn): Use them where we can't quit.