Add finger support to dirmngr.

The basic network code from http.c is used for finger. This keeps the
network related code at one place and we are able to use the somewhat
matured code form http.c. Unfortunately I had to enhance the http
code for more robustness and probably introduced new bugs.

Test this code using

gpg --fetch-key finger:wk@g10code.com

(I might be the last user of finger ;-)