Kernel+LibC: Add sys$disown() for disowning child processes

This syscall allows a parent process to disown a child process, setting
its parent PID to 0.

Unparented processes are automatically reaped by the kernel upon exit,
and no sys$waitid() is required. This will make it much nicer to do
spawn-and-forget which is common in the GUI environment.

Kernel/API/Syscall.h

@@ -194,7 +194,8 @@ namespace Kernel {
     S(sendfd)                 \
     S(recvfd)                 \
     S(sysconf)                \
-    S(set_process_name)
+    S(set_process_name)       \
+    S(disown)
 
 namespace Syscall {