bx509d: Add /get-tgts batch end-point

bx509d: Add /get-tgts batch end-point

In order to support batch jobs systems that run many users' jobs and
which jobs need credentials, we add a /get-tgts end-point that is a
batched version of the /get-tgt end-point.  This end-point returns JSON.

Also, we make GETs optional, default to not-allowed in preference of
POSTs.

We also correct handling of POST (before POSTs with non-zero-length bodies
would cause the server to close the connection), and add additional CSRF
protection features, including the ability to disable all GET requests
for /get-keys and /get-config.