Network: modify dnsmasq commandline build function to allow testing

Network: modify dnsmasq commandline build function to allow testing

The dnsmasq commandline was being built as a part of running
dnsmasq. This patch puts the commandline build into a separate
function (and exports it as a private API) making it possible to build
a dnsmasq commandline without executing it, so that we can write a
test program to verify that the proper commandlines are being created.

Signed-off-by: Michal Novotny <minovotn@redhat.com>