res_stasis_recording: Allow symbolic links in configured recordings dir.

If any component of ast_config_AST_RECORDING_DIR is a symbolic link we
would incorrectly assume the ARI user was trying to escape the recording
path.  Create additional check to check the recording directory's
realpath, only deny access if both do not match.

This is needed by the testsuite when run by 'run-local'.

Change-Id: I9145e841865edadcb5f75cead3471ad06bbb56c0