res_stir_shaken: Allow sending Identity headers for unknown TNs

Added a new option "unknown_tn_attest_level" to allow Identity
headers to be sent when a callerid TN isn't explicitly configured
in stir_shaken.conf.  Since there's no TN object, a private_key_file
and public_cert_url must be configured in the attestation or profile
objects.

Since "unknown_tn_attest_level" uses the same enum as attest_level,
some of the sorcery macros had to be refactored to allow sharing
the enum and to/from string conversion functions.

Also fixed a memory leak in crypto_utils:pem_file_cb().

Resolves: #921

UserNote: You can now set the "unknown_tn_attest_level" option
in the attestation and/or profile objects in stir_shaken.conf to
enable sending Identity headers for callerid TNs not explicitly
configured.

(cherry picked from commit 9e5cac457f)

res/res_stir_shaken/crypto_utils.c

@@ -498,11 +498,13 @@ static int pem_file_cb(const char *dir_name, const char *filename, void *obj)
 	if (lstat(filename_merged, &statbuf)) {
 		printf("Error reading path stats - %s: %s\n",
 					filename_merged, strerror(errno));
+		ast_free(filename_merged);
 		return -1;
 	}
 
 	/* We only want the symlinks from the directory */
 	if (!S_ISLNK(statbuf.st_mode)) {
+		ast_free(filename_merged);
 		return 0;
 	}
 
@@ -512,6 +514,7 @@ static int pem_file_cb(const char *dir_name, const char *filename, void *obj)
 		rc = crypto_load_store_from_cert_file(data->store, filename_merged);
 	}
 
+	ast_free(filename_merged);
 	return rc;
 }