added this behaviour and documentation

I'm playing a bit with clang-format(1). Not happy with the result yet, but some section are now legible, before were a mess 'cause it folded every line

Added the mx and my "special" values for MyMenu.x and MyMenu.y resources. These special values makes mymenu spawn under the mouse, like a contestual menu.

Clicking with Button1 select the item, Button3 is like C-m. Mentioned in the manpage also the undocumented scroll function. Mentioned the caveat that clicking past the last item is equivalent to clicking the last item.

1. remove the check_allocation macro (it was replaced with the idiom if ((x = ...) == NULL) err(1, "fn"); using the err(3) function instead of custom error-reporting. 2. added a break that was missing in the args parse