Optionally, don't start gpg-agent on start-up

Starting gpg-agent on start-up can be disabled by setting
KLEO_NO_GPG_AGENT_START=1. gpg-agent is started on demand by GnuPG and
Kleopatra so that it should be safe to disable the explicit start.

(cherry picked from commit 992201b0abff29d2cc6841683fc360d7632bde57)

• GnuPG-bug-id: T8181