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/exit/_Exit.c

@@ -1,9 +1,8 @@
 #include <stdlib.h>
-#define SYSCALL_NORETURN
 #include "syscall.h"
 
 void _Exit(int ec)
 {
-	syscall1(__NR_exit_group, ec);
-	syscall1(__NR_exit, ec);
+	__syscall(__NR_exit_group, ec);
+	__syscall(__NR_exit, ec);
 }