Various minor fixes (build on Fedora/RH, memory leaks)

This fixes a few unrelated issues:
   - Plugs memory leaks for elliptic curve and DH
   - Fix the value of the is_export flag for the dhparam_cb (it should be a boolean)
   - Adds useful context (either OpenSSL errors or the particular invalid option) to error messages across LuaSec
   - Fixes build of LuaSec on Fedora and RedHat (where OpenSSL is built without support for elliptic curves)