Dumb down some modified keypad keys in rxvt #3741
Description
In testing during #3440 it's been found that rxvt seems to stand apart from (almost?) all other terminals tested in that modified keys on the numeric keypad have their own escape sequences; so shift+- (on the numeric keypad, not top row -) results in an escape sequence, whereas in most other terminals it results in minus being emitted.
In this particular case, translate the sequences into the lowest-common-form of those keys.