Confirm that the PEM file paths that we'll be passing to the l_tls
object are loading Ok and request/validate the private key passphrase
if needed. Then also call eap_check_settings to validate the inner
method's settings.
The EAP-method's .probe methods only checked the method name so do that
in eap.c instead and allocate method state in .load_settings. Rename
method's .remove method to .free to improve the naming.
Only EAP as the inner authentication option is supported. According to
wikipedia this is the most popular EAP-TTLS use case, with MD5 as the
inner EAP's method.
