This was caused by a change to the upstream library
ProtonMail/go-crypto checking the flags on the keys more strictly.
However the signing key for rclone is very old and does not have those
flags. Adding those flags using `gpg --edit-key` and then the
`change-usage` subcommand to remove, save, quite then re-add, save
quit the signing capabilities caused the key to work.
This also adds tests for the verification and adds the selfupdate
tests into the integration test harness as they had been disabled on
CI because they rely on external sources and are sometimes unreliable.
Fixes#7373
This is possible now that we no longer support go1.12 and brings
rclone into line with standard practices in the Go world.
This also removes errors.New and errors.Errorf from lib/errors and
prefers the stdlib errors package over lib/errors.
