BACKPORT: 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).

BUG=chromium-os:38801
TEST=unit

Signed-off-by: Elly Fong-Jones <ellyjones@chromium.org>
(cherry picked from commit 254dc20bbaa68f955b1de8c7da6a02d52c786fd0)

Change-Id: I537037bd15d7c873f87e820a5291fc7a7e0ffca1
Reviewed-on: https://gerrit.chromium.org/gerrit/44075
Reviewed-by: Will Drewry <wad@chromium.org>
Commit-Queue: Elly Jones <ellyjones@chromium.org>
Tested-by: Elly Jones <ellyjones@chromium.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
6 files changed