List readline support in configure summary

* m4/readline.m4: Set gnupg_cv_have_readline.
* configure.ac: Add readline support to summary output.

Readline is an optional feature which is build if the readline
development files are available on the build systems. Too often they
are missing on a (new) build machine which at least makes debugging
inconvenient.
Backport useful code from fixes for bug 1447.

• configure.ac: Cehck for inet_ntop.
• m4/libcurl.m4: Provide a #define for the version of the curl

library.

We do not have keyserver helpers anymore but this fixes may come handy
eventually.