Make HKP keyserver engine work again.

We had some debug code here which prevented it from working.
The host selection code still needs a review!

• ks-engine-http.c (ks_http_help): Do not print help for hkp.
• ks-engine-hkp.c (ks_hkp_help): Print help only for hkp.

(send_request): Remove test code.
(map_host): Use xtrymalloc.

• certcache.c (classify_pattern): Remove unused variable and make

explicit substring search work.