hterm: change keymap for modifiers on Enter key
hterm: change keymap for modifiers on Enter key

hterm currently ignores Meta-Enter, Ctrl-Enter, etc. For arcane reasons,
if these combos are used in xterm, the window is maximized and the
program running in the shell does not receive them. When the keymap
code was refactored in feaf3143, it was modeled after xterm, and so it
ate Enter-with-modifiers and did not pass it through to applications.
Most terminal emulators today just escape these normally, and ignoring
them breaks some applications that users expect to be available.

Tested briefly with Secure Shell and the sed command, `sed -n l`, as
well as in emacs. Seems to behave the same as gnome-terminal.

Change-Id: I6b9a480001c8d363bebadf05cfd7383a07f0ef8a
Reviewed-on: https://chromium-review.googlesource.com/c/apps/libapps/+/1595147
Reviewed-by: Vitaliy Shipitsyn <6f51537d61e46a00bf958b60dbef5b5870c2bf22@google.com>
Tested-by: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>

chromium/hterm,chromium/hterm,chromium/hterm,chromium/hterm