Translate alternative start to M, also in variant

In case of an alternative start codon, the variant CDS was not
translated to a protein starting with M. This caused the protein
description machinery to conclude a variant affecting the start
codon, hence reporting `p.?`.

We fix this by always translating the start codon to M (except
when the variant actually affects it).

Example: `NM_024426.4:c.1107A>G` (a synomymous mutation) should
yield `NM_024426.4(WT1_i001):p.(=)`, not `p.?`. The start codon
for that protein is `CTG`.