Kernel: Introduce "sigaction" pledge

You now have to pledge "sigaction" to change signal handlers/dispositions. This
is to prevent malicious code from messing with assertions (and segmentation
faults), which are normally expected to instantly terminate the process but can
do other things if you change signal disposition for them.

Kernel/Process.cpp

@@ -2711,7 +2711,7 @@ int Process::sys$sigpending(sigset_t* set)
 
 int Process::sys$sigaction(int signum, const sigaction* act, sigaction* old_act)
 {
-    REQUIRE_PROMISE(stdio);
+    REQUIRE_PROMISE(sigaction);
     if (signum < 1 || signum >= 32 || signum == SIGKILL || signum == SIGSTOP)
         return -EINVAL;
     if (!validate_read_typed(act))

Kernel/Process.h

@@ -72,6 +72,7 @@ extern VirtualAddress g_return_to_ring3_from_signal_trampoline;
     __ENUMERATE_PLEDGE_PROMISE(video)   \
     __ENUMERATE_PLEDGE_PROMISE(accept)  \
     __ENUMERATE_PLEDGE_PROMISE(settime) \
+    __ENUMERATE_PLEDGE_PROMISE(sigaction) \
     __ENUMERATE_PLEDGE_PROMISE(shared_buffer)
 
 enum class Pledge : u32 {