TTY: Forward TIOCGPGRP from MasterPTY to SlavePTY

This makes tcgetpgrp() on a master PTY return the PGID of the slave PTY
which is probably what you are looking for. I'm not sure how correct or
standardized this is, but it makes sense to me right now.

Kernel/TTY/MasterPTY.cpp

@@ -98,7 +98,7 @@ void MasterPTY::close()
 
 int MasterPTY::ioctl(FileDescription& description, unsigned request, unsigned arg)
 {
-    if (request == TIOCSWINSZ)
+    if (request == TIOCSWINSZ || request == TIOCGPGRP)
         return m_slave->ioctl(description, request, arg);
     return -EINVAL;
 }