stir_shaken: Fix memory leak, typo in config, tn canonicalization

stir_shaken:  Fix memory leak, typo in config, tn canonicalization

* Fixed possible memory leak in tn_config:tn_get_etn() where we
weren't releasing etn if tn or eprofile were null.
* We now canonicalize TNs before using them for lookups or adding
them to Identity headers.
* Fixed a typo in stir_shaken.conf.sample.

Resolves: #716