Refactor: various: New force_host arg for pe_node_attribute_calculated()

We'll use this soon for querying an attribute from a bundle node's host
in colocations, regardless of whether XML_RSC_ATTR_TARGET is set to
"host".

Also implement best practices: Rename to pe__, add doxygen, improve
error checking.

Ref T489

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>