libkmod-signature: implement pkcs7 parsing with openssl

libkmod-signature: implement pkcs7 parsing with openssl

The patch adds data fetching from the PKCS#7 certificate using
openssl library (which is used by scripts/sign-file.c in the linux
kernel to sign modules).

In general the certificate can contain many signatures, but since
kmod (modinfo) supports only one signature at the moment, only first
one is taken.

With the current sign-file.c certificate doesn't contain signer
key's fingerprint, so "serial number" is used for the key id.

Signed-off-by: Yauheni Kaliuta <yauheni.kaliuta@redhat.com>