unifdef.1: typo: function-like arguments -> macros
unifdef: support function-like macro definitions

This is a fairly half-arsed implementation that treats function-like
macros the same as object-like macros that have been set to 1. It
is reasonably consistent with the existing half-arsed treatment
of function-like macro invocations in #if and #elif control expressions.

The manual now explains this half-arsedness more clearly.