Generate lists and use symbols in --libcurl code output.

This patch improves the output of curl's --libcurl option by
generating code which builds curl_httppost and curl_slist lists, and
uses symbolic names for enum and flag values.  Variants of the
my_setopt macro in tool_setopt.h are added in order to pass extra type
information to the code-generation step in tool_setopt.c.

If curl is configured with --disable-libcurl-option then the macros
call curl_easy_setopt directly.