Initialize terminal_state to terminal_is_ours

Initialize terminal_state to terminal_is_ours

Right now this variable is initialized to 0 i.e. terminal_is_inferior
and does not get set to terminal_is_ours until target_terminal_init() is
called.  This function however only gets called when an inferior is
first created.  In the meantime, terminal_state would wrongly remain set
to terminal_is_inferior.

Tested on x86_64 Debian Stretch -- native, gdbserver and
extended-gdbserver.

gdb/ChangeLog:

	* target.c (terminal_state): Initialize to terminal_is_ours.