New macros NULL_DEVICE and EXEC_SUFFIXES, to give the name of the

New macros NULL_DEVICE and EXEC_SUFFIXES, to give the name of the
	equivalent of /dev/null, and the suffixes used by executable
	files.  This is simple, and helps people porting Emacs to other
	operating systems.
	* process.h (NULL_DEVICE): Give this a default value.
	* process.c (Fstart_process): Pass EXEC_SUFFIXES to openp.
	(Fprocess_send_eof): Use NULL_DEVICE instead of "/dev/null".
	* callproc.c (Fcall_process): Pass EXEC_SUFFIXES to openp.
	Use NULL_DEVICE instead of "/dev/null".
	* s/vms.h (NULL_DEVICE): #define this.