Xref paths saved in a drawing (on open, on Reload, in the External
References list, XOPEN, nested xrefs) and table data-link paths re-read by
DATALINKUPDATE go through doc_path::resolve (craft-docpath): relative
paths resolve beside the drawing that stores them, and paths to other
computers, devices and URLs are not opened. Such an xref is Unresolved
with the reason, unless a file of its name is beside the host drawing.
Paths given to XATTACH, -XREF Path and DATALINK are used as given.
One load of xrefs reads at most 1 GB from referenced files in total.
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