gdb: rename scoped_gdb_tty_state, and make it non-copyable

gdb: rename scoped_gdb_tty_state, and make it non-copyable

The scoped_gdb_tty_state class seems misnamed.  For save/restore type
classes the pattern in GDB is usually scoped_restore_<whatever>, so
lets rename this to scoped_restore_tty_state.  I dropped the 'gdb' part
of the name as the underlying functions being called are
serial_get_tty_state and serial_set_tty_state, so the new name
matches (I think) what's actually being called.

I've also made the class non-copyable like other scoped_restore_
classes.

There should be no user visible changes after this commit.

Approved-By: Simon Marchi <simon.marchi@efficios.com>