vpn_manager: IpsecManager: Add XAUTH authentication

Add a new  flag "--xauth_credentials_file" which IpsecManager
uses to extract an XAUTH username and password.  With this
file supplied, strongSwan will be used for "xauthpsk" authentication
instead of "psk" and an XAUTH credential will be added to the
secrets file.

TEST=Unit test; upcoming VPNConnect/l2tpipsec_xauth

Change-Id: Ic1a0c62213f4cd6f6c336b1af27af2d3a59d3be8
Reviewed-on: https://chromium-review.googlesource.com/181130
Reviewed-by: Ben Chan <benchan@chromium.org>
Tested-by: Paul Stewart <pstew@chromium.org>
Commit-Queue: Paul Stewart <pstew@chromium.org>
4 files changed