Add key for toogling numlock handling

Add key for toogling numlock handling

Keypad will generate keycodes when keypad application mode is enabled. It
can cause problems with some programs like vi, which operates in such
mode.

This patch change by default don't generate the keycodes never, but this
behaviour can be changed using the combination Alt + NumLock.
---
 config.def.h |   34 ++++++++++++++++++----------------
 st.c         |   17 +++++++++++++++--
 2 files changed, 33 insertions(+), 18 deletions(-)