Refactor: libcrmcommon: Add pcmk__xe_foreach_child.

This function can be used to iterate over all children of a given XML
node with a given name. It can also be used to iterate over all
children of a given XML node if no name is given. This can be used to
replace all the first_named_child/crm_next_same_xml loops, as well as
the pcmkxe_first_child/pcmkxe_next loops.

Fixes T525