signature: implement support for subpackets.

The subpacket types currently supported are creation time and issuer
key id. Both hashed data and unhashed data is parsed, but note that
the code currently does not handle them differently.
libsign: error out when given zero-sized buffers.

Previously, the code would incorrectly return 0 when given zero sized
buffers when parsing signatures or public keys.
Merge branch 'bugfix-zero_sized_buffers' of /home/git/repositories/root/x-18789-libsign
cmake/modules: support MSVC debug builds of GMP.

This adds support for debug builds of GMP/MPIR using MSVC. It also adds
a couple of variables to explicitly locate the GMP DLL and library dir.
Mostly used for windows stuff.
libsign: add CMake package files.

This adds cmake package files so we can easier find the library when
using CMake for other projects.
Merge branch 'feature-cmake_package_files' of /home/git/repositories/root/x-18789-libsign