Refactor: scheduler: functionize finding config for exact action key

Previously, find_rsc_op_entry_helper() used a goto to retry finding an action
via alternative names. Now, it calls a new function find_exact_action_config().
This is easier to follow and will make planned changes easier.

While we're at it, add doxygen blocks, use bool instead of gboolean, use
convenience functions where appropriate, compare action keys case-sensitively,
and improve variable names, comments, and formatting.