Fix restoring of inferior terminal settings

Fix restoring of inferior terminal settings

I noticed that the child_terminal_save_inferior function was not used
since the commit f6ac5f3d63e0 ("Convert struct target_ops to C++").  I
was able to make a little test program to illustrate the problem (see
test case).

I think we're just missing the override of the terminal_save_inferior
method in inf_child_target (along with the other terminal-related
methods).

Instead of creating a new test, I thought that gdb.base/term.exp was a
good candidate for testing that gdb restores properly the inferior's
terminal settings.

gdb/ChangeLog:

	* inf-child.h (inf_child_target) <terminal_save_inferior>: New.
	* inf-child.c (inf_child_target::terminal_save_inferior): New.

gdb/testsuite/ChangeLog:

	* gdb.base/term.exp: Compare terminal settings with values from
	the inferior.
	* gdb.base/term.c: Get and set terminal settings.