(vc-bzr-registered): Gracefully handle missing "bzr"

(vc-bzr-registered): Gracefully handle missing "bzr"
program, and return nil
(vc-bzr-state): Gracefully handle missing "bzr" program, and return nil.
(vc-bzr-state): Look for path names relative to the repository
root after status keyword.
(vc-bzr-file-name-relative): New function.
(vc-bzr-admin-dirname): Reinstate, as other vc-bzr-admin-... paths
depend on it.
(vc-bzr-admin-dirname, ...-checkout-format-file)
(...-branch-format-file, ...-revhistory): Paths to some Bzr internal
files that we now parse directly for speed.
(vc-bzr-root-dir): Use `vc-bzr-admin-checkout-format-file' as witness.
(vc-bzr-registered): Only parse vc-bzr-admin-dirstate file if it exists.
(vc-bzr-state): "bzr status" successful only if exitcode is 0
(vc-bzr-root): Use `vc-bzr-shell-command'.  Stderr may contain
Bzr warnings, so we must discard it.
(vc-bzr-workfile-version): Speedup counting lines from
`vc-bzr-admin-revhistory' file, but fallback to spawning "bzr revno"
if that file doesn't exist.
(vc-bzr-responsible-p): Use `vc-bzr-root' instead of
`vc-bzr-root-dir' for speed.
add `vc-bzr-admin-dirname' (not ".bzr"!) to `vc-directory-exclusion-list'
(vc-bzr-shell-command): New function.