Use Bob Jenkins lookup3 hash instead of FNV.

Use Bob Jenkins lookup3 hash instead of FNV.

The Jenkins lookup3 hash is superior to FNV in collision resistance,
avalanching, and performance on systems that do not have fast
multiplication.  It also provides a good way to combine the result of
a previous hashing step with the current hash, using its "basis" argument.
This commit replaces the PSPP implementation of FNV with the Jenkins
lookup3 hash and updates all the current users.

In addition, John Darrington pointed out that commit dd2e61b4a
"Make create_iconv() properly distinguish converters by name"
unintentionally introduced gratuitous hash collisions, by causing
all converters where tocode and fromcode were the same to hash to
value 0, and converters where tocode and fromcode were swapped to
hash to the same value as each other.  Using the "basis" argument to
the Jenkins hash properly, instead of just attempting to combine
hash values with XOR, fixes this problem.