Don't propagate our current terminal state to the inferior

Don't propagate our current terminal state to the inferior

Currently when we start an inferior we have the inferior inherit our
terminal state.  Under TUI, our terminal is highly modified by ncurses
and readline.  So when starting an inferior under TUI, the inferior will
have a highly modified terminal state which will interfere with standard
I/O. For example,

$ gdb gdb
(gdb) break main
(gdb) run
(gdb) print puts ("a\nb")
a
b
$1 = 4
(gdb) [enter TUI mode]
(gdb) run
(gdb) [exit TUI mode]
(gdb) print puts ("a\nb")
a
 b
  $2 = 4
(gdb) print puts ("a\r\nb\r")
a
b
$3 = 6

As you can see, when we start the inferior under the regular interface,
puts() prints the text properly.  But when we start the inferior under
TUI, puts() does not print the text properly.  This is because when we
start the inferior under TUI it inherits our current terminal state
which has been modified by ncurses to, among other things, require an
explicit \r\n to print a new line. As a result the inferior performs
standard I/O in an unexpected way.

Because of this discrepancy, it doesn't seem like a good idea to have
the inferior inherit our _current_ terminal state for it may have been
modified by readline and/or ncurses.  Instead, we should have the
inferior inherit a pristine snapshot of our terminal state taken before
readline or ncurses have had a chance to alter it.  This enables the
inferior to run in a more accurate way, more closely mimicking the
program's behavior had it run standalone.  And it fixes the above
mentioned issue.

Tested on x86_64-unknown-linux-gnu.

gdb/ChangeLog:

	* terminal.h (set_initial_gdb_ttystate): Declare.
	* inflow.c (initial_gdb_ttystate): New static variable.
	(set_initial_gdb_ttystate): New setter.
	(child_terminal_init_with_pgrp): Copy initial_gdb_ttystate
	instead of our current terminal state.
	* top.c (gdb_init): Call set_initial_gdb_ttystate.