Call nonl before wgetch in TUI

PR tui/28819 points out that, in the TUI, the C-j and C-m keys cannot
be bound differently in one's ~/.inputrc.  However, this works in
other readline applications.

The bug is that the TUI uses curses' "nl" mode, which causes wgetch to
return the same value for both keys.  There is a "nonl" mode, but it
also affects output.

This patch fixes the bug by arranging to call nonl before reading a
key and then nl afterward.  This avoids any potential problem with
changing the output if gdb was to use nonl globally.

gdb/ChangeLog
2018-10-06  Tom Tromey  <tom@tromey.com>

	PR tui/28819:
	* tui/tui-io.c (gdb_wgetch): New function.
	(tui_mld_getc, tui_getc): Use it.