syscall overhaul part two - unify public and internal syscall interface

with this patch, the syscallN() functions are no longer needed; a
variadic syscall() macro allows syscalls with anywhere from 0 to 6
arguments to be made with a single macro name. also, manually casting
each non-integer argument with (long) is no longer necessary; the
casts are hidden in the macros.

some source files which depended on being able to define the old macro
SYSCALL_RETURNS_ERRNO have been modified to directly use __syscall()
instead of syscall(). references to SYSCALL_SIGSET_SIZE and SYSCALL_LL
have also been changed.

x86_64 has not been tested, and may need a follow-up commit to fix any
minor bugs/oversights.

src/signal/sigtimedwait.c

@@ -8,7 +8,7 @@ int sigtimedwait(const sigset_t *mask, siginfo_t *si, const struct timespec *tim
 	int ret;
 	CANCELPT_BEGIN;
 	do {
-		ret = syscall4(__NR_rt_sigtimedwait, (long)mask, (long)si, (long)timeout, SYSCALL_SIGSET_SIZE);
+		ret = syscall(__NR_rt_sigtimedwait, mask, si, timeout, 8);
 	} while (ret<0 && errno==EINTR);
 	CANCELPT_END;
 	return ret;