First, it was not easily possible to set a private key without
also providing a password.  This used to be OK, but now with
secret flags it may be the case that when the connection is read,
there's no private key password.  So functions that set the
private key must account for NULL passwords.

Unfortunately, the crytpo code did not handle this case well.
We need to be able to independently (a) verify that a file looks
like a certificate or private key and (b) that a given password
decrypts a private key.  Previously the crypto code would fail
to verify the file when the password was NULL.

So this change fixes up the crytpo code for a more distinct
split between these two operations, such that if no password is
given, the file is still checked to ensure that it's a private
key or a certificate.  If a password is given, the password is
checked against the private key file.

This commit also changes how private keys and certificates were
handled with the BLOB scheme.  Previously only the first certificate
or first private key was included in the property data, while now
the entire file is encoded in the data.  This is intended to fix
cases where multiple private keys or certificates are present in
a PEM file.  It also allows clients to push certificate data to
NetworkManager for storage in system settings locations, which was
not as flexible before when only part of the certificate or key
was sent as the data.