unifdef(1): Improve worst-case bound on symbol resolution

Use RB_TREE to make some algorithms O(lg N) and O(N lg N) instead of O(N)
and O(N^2). Because N is typically small and the former linear array also has
great constant factors (as a property of CPU caching), this doesn't provide
material benefit most or all of the time.

While here, remove arbitrarily limit on number of macros understood.