Update to github.com/mtrmac/gpgme v0.1.2

This fixes CVE-2020-8945 by incorporating proglottis/gpgme#23 .

Other changes included by the rebase:
- Support for gpgme_off_t (~no-op on Linux)
- Wrapping a few more GPGME functions (irrelevant if we don't call them)

Given how invasive the CVE fix is (affecting basically all binding
code), it seems safer to just update the package (and be verifiably
equivalent with upstream) than to backport and try to back out the few
other changes.

Performed by
$ go get github.com/mtrmac/gpgme@v0.1.2
$ make vendor

Signed-off-by: Miloslav Trmač <mitr@redhat.com>

go.mod

@@ -9,6 +9,7 @@ require (
 	github.com/docker/docker v0.0.0-20180522102801-da99009bbb11
 	github.com/dsnet/compress v0.0.1 // indirect
 	github.com/go-check/check v0.0.0-20180628173108-788fd7840127
+	github.com/mtrmac/gpgme v0.1.2 // indirect
 	github.com/opencontainers/go-digest v1.0.0-rc1
 	github.com/opencontainers/image-spec v1.0.2-0.20190823105129-775207bd45b6
 	github.com/opencontainers/image-tools v0.0.0-20170926011501-6d941547fa1d

go.sum

@@ -91,6 +91,8 @@ github.com/mistifyio/go-zfs v2.1.1+incompatible h1:gAMO1HM9xBRONLHHYnu5iFsOJUiJd
 github.com/mistifyio/go-zfs v2.1.1+incompatible/go.mod h1:8AuVvqP/mXw1px98n46wfvcGfQ4ci2FwoAjKYxuo3Z4=
 github.com/mtrmac/gpgme v0.0.0-20170102180018-b2432428689c h1:xa+eQWKuJ9MbB9FBL/eoNvDFvveAkz2LQoz8PzX7Q/4=
 github.com/mtrmac/gpgme v0.0.0-20170102180018-b2432428689c/go.mod h1:GhAqVMEWnTcW2dxoD/SO3n2enrgWl3y6Dnx4m59GvcA=
+github.com/mtrmac/gpgme v0.1.2 h1:dNOmvYmsrakgW7LcgiprD0yfRuQQe8/C8F6Z+zogO3s=
+github.com/mtrmac/gpgme v0.1.2/go.mod h1:GYYHnGSuS7HK3zVS2n3y73y0okK/BeKzwnn5jgiVFNI=
 github.com/mtrmac/image/v4 v4.0.0-20191002203927-a64d9d2717f4 h1:AE5cilZfrGtAgMg5Ed4c2Y2KczlOsMVZAK055sSq+gc=
 github.com/mtrmac/image/v4 v4.0.0-20191002203927-a64d9d2717f4/go.mod h1:0ASJH1YgJiX/eqFZObqepgsvIA4XjCgpyfwn9pDGafA=
 github.com/mtrmac/image/v4 v4.0.0-20191003181245-f4c983e93262 h1:HMUEnWU3OPT09JRFQLn8VTp3GfdfiEhDMAEhkdX8QnA=