Overload the certificate and key properties to allow paths to the
certificates and keys using a special prefix for the property data.
Add API to libnm-util for easy certificate path handling, and
documentation for NMSetting8021x.
The only thing that doesn't work yet is the system-settings service's
"auto eth" connections for ethernet devices that don't have an existing
connection. Might also have issues with unmanaged devices that can't
provide a MAC address until they are brought up, but we'll see.
Plugins no longer need to hash WPA passphrases, so there's no need to keep
sha1 stuff around unless its for hasing other stuff (ifcfg-rh uses sha1
for certificate hashing for example, but has a private copy).
TLS uses the 'identity' which previously wasn't read. The private key
password should also only be used for PKCS#12 files, becuase they aren't
decrypted when read into the setting.
Private keys also need to be handled differently; PKCS#12 keys are written
out unchanged (ie, still encrypted) with their corresponding private key.
DER keys are stored in the setting unencrypted, so they are re-encrypted
before being written out to disk. But because the private key password
isn't known for DER keys, a random password must be used to re-encrypt
the key.
