--libcurl: fix for non-zero default options

If the default value for an option taking a long as its value is non
zero, and it is set by zero by a command line option, then that command
line option is not reflected in --libcurl's output. This is because line
520-521 of tool_setopt.c look like:

if(!lval)
    skip = TRUE;

An example of a command-line option doing so is the -k option that sets
CURLOPT_SLL_VERIFYPEER and CURLOPT_SSL_VERIFYHOST to 0L, when the
defaults are non-zero.