Add an agentx_retry call.
If a session, agentcaps, region, index, or object doesn't properly open
because of an error returned by the AgentX master it remained closed until
a reconnect. This new function call walks the tree and tries to reopen
everything in a closed state.

OK sthen@

s/agentx_/ax_/g and s/subagentx_/agentx_/g

Requested by and OK deraadt@

Import libagentx.

This is the same code as I committed earlier to relayd with some fixes by
bluhm@. Not yet linked to the build so developers can return home before
things explode.
To be used by other daemons soon(tm).

OK bluhm@