support kernels with no SYS_open syscall, only SYS_openat

open is handled specially because it is used from so many places, in
so many variants (2 or 3 arguments, setting errno or not, and
cancellable or not). trying to do it as a function would not only
increase bloat, but would also risk subtle breakage.

this is the first step towards supporting "new" archs where linux
lacks "old" syscalls.

src/misc/realpath.c

@@ -22,7 +22,7 @@ char *realpath(const char *restrict filename, char *restrict resolved)
 		return 0;
 	}
 
-	fd = syscall(SYS_open, filename, O_PATH|O_NONBLOCK|O_CLOEXEC|O_LARGEFILE);
+	fd = sys_open(filename, O_PATH|O_NONBLOCK|O_CLOEXEC);
 	if (fd < 0) return 0;
 	__procfdname(buf, fd);