proposal: Make sure to consider all transform types when selecting proposals

proposal: Make sure to consider all transform types when selecting proposals

This way there will be a mismatch if one of the proposals contains
transform types not contained in the other (the fix list of transform
types used previously resulted in a match if unknown transform types
were contained in one of the proposals).  Merging the sets of types
makes comparing proposals with optional transform types easier (e.g.
DH for ESP with MODP_NONE).