Substitute `certificate' to `public-key' in `(gnutls extra)'.
* guile/modules/gnutls/build/enums.scm (%openpgp-key-format-enum):
Rename to...
(%openpgp-certificate-format-enum): This.
(%gnutls-extra-enums): Update.
* guile/modules/gnutls/build/smobs.scm (%openpgp-public-key-smob):
Rename to...
(%openpgp-certificate-smob): This.
(%gnutls-extra-smobs): Update.
* guile/modules/gnutls/extra.scm: Substitute all `certificate' in all
`public-key' names. Add backward-compatible bindings.
* guile/src/extra.c: Substitute `certificate' to `public-key'.