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/Process.cpp

@@ -2716,3 +2716,9 @@ int Process::sys$dump_backtrace()
     dump_backtrace();
     return 0;
 }
+
+int Process::sys$dbgputch(u8 ch)
+{
+    IO::out8(0xe9, ch);
+    return 0;
+}

Kernel/Process.h

@@ -104,6 +104,7 @@ public:
     void die();
     void finalize();
 
+    int sys$dbgputch(u8);
     int sys$dump_backtrace();
     int sys$gettid();
     int sys$donate(int tid);

Kernel/Syscall.cpp

@@ -70,6 +70,8 @@ static u32 handle(RegisterDump& regs, u32 function, u32 arg1, u32 arg2, u32 arg3
     case Syscall::SC_putch:
         Console::the().put_char(arg1 & 0xff);
         break;
+    case Syscall::SC_dbgputch:
+        return current->process().sys$dbgputch((u8)arg1);
     case Syscall::SC_sleep:
         return current->process().sys$sleep((unsigned)arg1);
     case Syscall::SC_usleep:

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 {