Low: attrd,lrmd,liblrmd,libservices: simplify lrmd alert execution

Previously, attrd used lrmd to execute alerts by registering alert agents as
resources, and using the usual resource execution API. The crmd calls the
services library directly, but still uses a function intended for resources.
Alerts don't need the same level of tracking as resources, so these methods are
unnecessarily complicated.

This revision simplifies the process by adding new services library functions
for creating and executing an alert agent action. Instead of registering the
alert agent, the client now passes the alert agent path to the lrmd alert
execution API, and the lrmd's handler for that API does a "fire and forget" by
calling the new functions.