download.pl: use curl in preference to wget

Because wget doesn't know how to do Negotiate authentication with a proxy
and curl does, use curl if it's present. The user is expected to have a
~/.curlrc that sets the options necessary for any proxy authentication.

A ~/.curlrc is completely optional however and curl will work in exactly
the same manner as wget without one.

Signed-off-by: Brian J. Murrell <brian@interlinx.bc.ca>
[Jo-Philipp Wich: Rework code to detect curl usability by checking --version,
                  Use vararg style open() to bypass the shell when downloading,
                  Use Text::ParseWords to decompose env vars into arguments]
Signed-off-by: Jo-Philipp Wich <jo@mein.io>