Implement sending signals to blocked-in-kernel processes.

This is dirty but pretty cool! If we have a pending, unmasked signal for
a process that's blocked inside the kernel, we set up alternate stacks
for that process and unblock it to execute the signal handler.

A slightly different return trampoline is used here: since we need to
get back into the kernel, a dedicated syscall is used (sys$sigreturn.)

This restores the TSS contents of the process to the state it was in
while we were originally blocking in the kernel.

NOTE: There's currently only one "kernel resume TSS" so signal nesting
definitely won't work.

Userland/sh.cpp

@@ -331,8 +331,12 @@ int main(int, char**)
         char keybuf[16];
         ssize_t nread = read(0, keybuf, sizeof(keybuf));
         if (nread < 0) {
-            printf("failed to read :(\n");
-            return 2;
+            if (errno == EINTR) {
+                // Ignore. :^)
+            } else {
+                perror("read failed");
+                return 2;
+            }
         }
         for (ssize_t i = 0; i < nread; ++i) {
             putchar(keybuf[i]);