docs: Implement our own dot->pic translator.

docs: Implement our own dot->pic translator.

Recent versions of Graphviz no longer support output to PIC format, so this
commit adds our own internal translator from dot's "plain" output format
to PIC format.  The "plain" format works best with slightly different
"dot" input (advised by the Graphviz manual description of the "plain"
format) so this commit also adjusts ovsdb-dot's output.