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/thread/pthread_cancel.c

@@ -1,7 +1,6 @@
-#define SYSCALL_RETURN_ERRNO
 #include "pthread_impl.h"
 
 int pthread_cancel(pthread_t t)
 {
-	return syscall3(__NR_tgkill, t->pid, t->tid, SIGCANCEL);
+	return pthread_kill(t, SIGCANCEL);
 }