change hotkeys to adjust gaps to fit in the grand scheme
fix a badly indented else-if
add the ability to change the border size on the fly

Includes addition of a global borders variable, changing all layout
calculations to use it instead of BORDER_WIDTH and adding the default
shortcuts/manpage entries.
refactor conditions in adjust_borders/_gaps
coding style fix
fix windows not re-tiling properly when adjusting border size
Merge branch 'varborders'

Conflicts:
	frankenwm.c