Add some command-line options to ctdb_diagnostics.

Add some command-line options to ctdb_diagnostics.

In some contexts ctdb_diagnostics generates too many errors when it is
run on heterogeneous and machine-configured clusters.  In some
clusters some nodes are expected to be differently configured and also
machine-generated configured files can have comments containing
timestamps.

This adds some command-line options that can be used to reduce the
number of errors reported:

    -n <nodes>  Comma separated list of nodes to operate on
    -c          Ignore comment lines (starting with '#') in file comparisons
    -w          Ignore whitespace in file comparisons
    --no-ads    Do not use commands that assume an Active Directory Server

The -n option simply allows ctdb_diagnostics to operate on a subset of
nodes, avoiding file comparisons with and data collection on nodes
that are differently configured.  For file comparisons, instead of
showing each file on the current node and then comparing other nodes
to that file, the file from the first (available or requested) nodes
is shown and then other nodes are compared to that.  That has resulted
in changes in output - that is, ctdb diagnostics no longer prints
messages referencing the current node.

-c and -w are used to weaken comparisons between configuration files.

--no-ads can be used to avoid running ADS-specific commands if a
cluster uses LDAP (or other non-ADS) configuration.

This also fixes a number of bugs in related code:

* A call to onnode was losing the >> NODE ...  << lines because they
  now go to stderr.  This was changed in onnode long ago but
  ctdb_diagnostics was never updated to match.

* ctdb_diagnostics was counting lines in /etc/ctdb/nodes to determine
  what nodes to operate on.  For some time the nodes file has
  supported syntax that makes this invalid.  "ctdb listnodes -Y" is
  now used to list available nodes.

Signed-off-by: Martin Schwenke <martin@meltin.net>