fix: uniformly treat margin as unsigned in pack.c

Matthew Fernandezrequested to merge
smattr/graphviz:b2108122-f2f7-4e5a-9d70-52681500d63a into master

Although margin is an unsigned int in pack_info, it was being passed around as a signed int within pack.c. UBSan identified seven undefined overflows that happened as a result of this with the following input:

digraph G {rLn pack=-11335142740g}er-

Fixes #1681 (closed). Several of these were also pointed to by -Wsign-conversion compiler warnings.

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