authorized_keys.py: find key type by matching against blob prefix
authorized_keys.py: find key type by matching against blob prefix

Resolves the possible ambiguity in the old method used to distinguish
between options and key type. However, the parser now rejects lines that
have mismatched types (i.e. plaintext type different from the one in key
blob).

grawity/hacks,grawity/hacks,grawity/hacks,grawity/hacks,grawity/hacks,grawity/hacks,grawity/hacks