tlsdated: support command-line proxy override

This makes a proxy passed on the command line with -x supplant proxies for
configured sources, and makes the 'proxy' stanza in sources optional (in which
case we supply no proxy argument for them).

Signed-off-by: Elly Fong-Jones <ellyjones@chromium.org>
Merge pull request #77 from elly/tlsdated-proxy

tlsdated: support command-line proxy override