mutt: add a couple of helper functions to filters
mutt: add a couple of helper functions to filters

Still more to do here; this is just the first step in a series of
refactors.

wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent