Add KEYMAP_INVALID value for enum keymap_id and properly check for it

Add KEYMAP_INVALID value for enum keymap_id and properly check for it

get_keymap_id returns -1 when it can't find the keymap.  Because the return
type of get_keymap_id is enum keymap_id and enum keymap_id did not have any
explicit values defined, it could be unsigned, which meant that when
get_keymap_id returned -1, it was really returning a huge positive number.
This meant that when callers checker whether the return value was negative,
they were essentially performing no check at all, so they might give
get_keymap_id an invalid keymap name, get back an invalid keymap_id, and
use that invalid keymap_id.

This commit adds KEYMAP_INVALID = -1 to enum keymap_id and makes all
functions that deal with the enumeration use that symbol.