Merged mostly completed `be email-bugs'.

Merged mostly completed `be email-bugs'.

Highlights:
 * new be commands 'email-bugs' and 'import-xml'
 * standardized <be-xml> format for XML files.
 * new be-handle-mail interface '[be-bug:xml]'
 * restrict_file_access security patch
 * new subprocess handling submodule libbe.subproc
 * test.py adjusted to use an installed VCS for most tests.
 * assorted bugfixes

Altered interfaces to the following be commands:
 * comment --xml     tag gone, use import-xml.
 * show --xml        xml format updated to <be-xml> format.

Also adjusted be-mbox-to-xml and be-xml-to-mbox to handle new <be-xml>
format and provide better handling of *.extra_strings.