gpgparsemail: Allow weirdly-mixed pkcs7 signatures.

* tools/gpgparsemail.c: Add and check info->signing_protocol_2.

Some mailers in the wild will generate messages that have the a weird
structure where they use the x- prefix in one part and drop it in
another.

For example, the main MIME part as a whole has:

Content-Type: multipart/signed;

protocol="application/x-pkcs7-signature"

but the signature sub-part has:

Content-Type: application/pkcs7-signature

(or vice versa, where the x- prefix is in the sub-part but not the
protocol= section on the main MIME object)

This change also avoids allocating strings for these comparisons,
since the const strings in question are already available in the built
executable, and no dynamic labels are needed.

• 2 lines reformatted to keep them below 90 cols. - wk