Kernel+LibC: Add a dbgputch() syscall and use it for userspace dbgprintf().

The "stddbg" stream was a cute idea but we never ended up using it in
practice, so let's simplify this and implement userspace dbgprintf() on top
of a simple dbgputch() syscall instead.

This makes debugging LibC startup a little bit easier. :^)

Kernel/Syscall.h

@@ -116,7 +116,8 @@ struct timeval;
     __ENUMERATE_SYSCALL(fchown)                 \
     __ENUMERATE_SYSCALL(halt)                   \
     __ENUMERATE_SYSCALL(reboot)                 \
-    __ENUMERATE_SYSCALL(dump_backtrace)
+    __ENUMERATE_SYSCALL(dump_backtrace)         \
+    __ENUMERATE_SYSCALL(dbgputch)
 
 namespace Syscall {