write_file(): drop "fatal" parameter

All callers except three passed 1 for the "fatal" parameter to ask
this function to die upon error, but to a casual reader of the code,
it was not all obvious what that 1 meant.  Instead, split the
function into two based on a common write_file_v() that takes the
flag, introduce write_file_gently() as a new way to attempt creating
a file without dying on error, and make three callers to call it.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

daemon.c

@@ -1376,7 +1376,7 @@ int main(int argc, char **argv)
 		sanitize_stdfds();
 
 	if (pid_file)
-		write_file(pid_file, 1, "%"PRIuMAX"\n", (uintmax_t) getpid());
+		write_file(pid_file, "%"PRIuMAX"\n", (uintmax_t) getpid());
 
 	/* prepare argv for serving-processes */
 	cld_argv = xmalloc(sizeof (char *) * (argc + 2));