ChromeOS: Add more host verification options for OpenVpn.

Support the --verify-hash and --verify-x509-name options of OpenVPN via ONC.

Note that the ONC validator restricts the possible types passable to --verify-x509-name to "name", "name-prefix" and "subject". See also https://community.openvpn.net/openvpn/wiki/Openvpn23ManPage .

BUG=276291
R=armansito@chromium.org, bartfab@chromium.org, pstew@chromium.org

Review URL: https://codereview.chromium.org/61903002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@234495 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 234495