Prefer gpgme_user_id_t's address field over the email field

GMimeCertificate's email field is typically populated from the email field of the highest-validity gpgme_user_id_t object.

But gpgme offers some confusing commentary on the gpgme_user_id_t, in particular:

char *email
  This is the email component of uid, if available.
char *address;
  The mail address (addr-spec from RFC-5322) of the user ID
  string. This is general the same as the email part of this
  structure but might be slightly different. If no mail address is
  available NULL is stored.

It's hard to say exactly what this difference means.

Experimenting with some certificates and gpgme 1.15.1, it looks to me like the differences between email and address are:

when an OpenPGP User ID does contain an e-mail address, the two fields appear to be identical: they are both the same addr-spec.
when an OpenPGP User ID does not appear to contain an e-mail address, email is set to the empty string, but address is set to NULL.
when rendering the X.509 Subject DN as a user ID, the email is the the empty string, but the address is set to NULL (this is true regardless of whether the DN contains an 1.2.840.113549.1.9.1 ("emailAddress") component, which I've suggested might change, but I'm not convinced anything will happen).
when rendering an X.509 SubjectAltName of type rfc822Name, address is an addr-spec, but "email" is an angle-addr. that is, for Alice's signing cert, the address field is alice@smime.example but the email field is <alice@smime.example>
when rendering an X.509 SubjectAltName of any other type, email is the empty string but address is NULL.

It looks to me like address gives us a more stable guarantee -- it's either NULL or an addr-spec, while email might vary between:

the empty string
addr-spec
angle-addr

(maybe the email field could be NULL as well, but i haven't encountered that yet).

This patch prefers to populate the email field of GMimeCertificate from gpgme_user_id_t's address field, when it is present but differs from the email field.

jstedfast / gmime

Prefer gpgme_user_id_t's address field over the email field #102