Fix: libcib: Prevent based or cibadmin from crashing when handling an XPath query for an attribute

[gao-yan]

With 5fe98f4 in 3.0.1, if an XPath query for an attribute resulted in a
single match, for example:

$ cibadmin --query --xpath "/cib/@crm_feature_set"

, based would encounter a fatal assertion in pcmk__xml_copy():

error: pcmk__xml_copy: Triggered fatal assertion at xml.c:844 : src->type == XML_ELEMENT_NODE

The assertion wouldn't be triggered if there were multiple matches for an
attribute, despite the fact that only the last match would be displayed:

$ cibadmin --query --xpath "//@uname"
<xpath-query uname="node2"/>

But in case --node-path option was anyhow specified, cibadmin would
encounter a fatal assertion in output_xml_id():

error: output_xml_id: Triggered fatal assertion at cibadmin.c:865 : id != NULL

If --no-children option was anyhow specified, the attribute name would be
displayed as an element name and the value wouldn't be shown:

$ cibadmin --query --xpath "/cib/@crm_feature_set" --no-children
<crm_feature_set/>

This commit fixes the issues by handling non-element matches separately.

Besides, instead of being displayed with a vague element name "xpath-query",
now a matching attribute is displayed with the corresponding element name
and the ID if present, so that the output is meaningful:

$ cibadmin --query --xpath "/cib/@crm_feature_set"
<cib crm_feature_set="3.20.5"/>

Multiple matches are all displayed:

$ cibadmin --query --xpath "//@uname"
<xpath-query>
  <node id="1" uname="node1"/>
  <node id="2" uname="node2"/>
  <node_state id="1" uname="node1"/>
  <node_state id="2" uname="node2"/>
</xpath-query>

Also, although cibadmin --node-path option is deprecated, the previous
behavior is restored by handling the corresponding element of a
non-element match instead of returning nothing:

$ cibadmin --query --xpath "//@uname" --node-path
/cib/configuration/nodes/node[@id='1']
/cib/configuration/nodes/node[@id='2']
/cib/status/node_state[@id='1']
/cib/status/node_state[@id='2']

And --node-path option is respected even if --no-children is also specified.