Previously, if the header was present, we'd proceed even if the
function wasn't there.
Easy fix for bug 19161. A better fix would involve trying harder to
find libscrypt_scrypt.
This marks some lines as unreachable by the unit tests, and as
therefore excluded from test coverage.
(Note: This convention is only for lines that are absolutely
unreachable. Don't use it anywhere you wouldn't add a
tor_fragile_assert().)
This bug shouldn't be reachable so long as secret_to_key_len and
secret_to_key_make_specifier stay in sync, but we might screw up
someday.
Found by coverity; this is CID 1241500
Uses libscrypt when found; otherwise, we don't have scrypt and we
only support openpgp rfc2440 s2k hashing, or pbkdf2.
Includes documentation and unit tests; coverage around 95%. Remaining
uncovered code is sanity-checks that shouldn't be reachable fwict.
