Introduce header_add and header_unset

This commit squashes commits 53c2e28...3588a9a

Credits go to Olivier Favre (@ofavre) and Hauke Lampe (@lampeh)
who did all the hard work

This is the code from with the provided feedback incorporated:
https://github.com/varnish/libvmod-curl/pull/1
https://github.com/varnish/libvmod-curl/pull/2