golang/x/crypto/openpgp is frozen. This commit switches to ProtonMail's fork
github.com/ProtonMail/go-crypto/openpgp to have a version that is used and
maintained by the upstream project.
Move X.509 and Openpgp operations into custom packages. Implement more
robust input reading. Do not convert []byte to string unnecessarily.
Finish implementation of X.509 CRL creation.
Add a main loop, move I/O code into io.go, move configuration into config.go.
Use shared.Decode24BitLength instead of manually decoding block lengths.
Fix response block decoding and checksum validation.
Add constants for commonly used byte values and use these in the signer and
the client.
The client can now talk to the old Perl signer implementation. Running
socat has been documented in README.md. Commonly used I/O code has been
moved to the shared/io.go file.
