Extend NSData+Crypto to support PBKDF2 with SHA1 HMAC as dovecot
is using it since v2.3.0.
The format hashed passwords is {PBKDF2}$1$<salt>$<rounds>$<hashed value in hex>
The implementation of pkcs#5 PBKDF2 is taken from openbsd (with minor
adjustments) as OpenSSL and GnuTLS would require quite new versions to
support this hash.
