connection: Fall through to global module triggers

Make module connection pool triggers use global module trigger
configuration, if there is no "trigger" section in the pool section.

Use fully-qualified module-specific trigger names for module-specific
connection pools in connection.c.

E.g. trigger "modules.ldap.open", instead of just "open" for pools
initialized with fr_connection_pool_module_init, being passed "ldap"
config section.

Send triggers even if the pool has no "trigger" section.

This makes exec_trigger fall through to global module triggers, if the
pool configuration doesn't have the "trigger" section.