gpg-agent connection count: Abstract away all direct accesses

gpg-agent connection count: Abstract away all direct accesses

This variable needs to be protected with a mutex.  We don't want
anyone to access it directly.  So introduce a helper function
adjust_agent_active_connections to increment/decrement it, and replace
all direct accesses.

Also replace a couple of direct read-only accesses with calls to the
existing function get_agent_active_connection_count.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>