add a '--filepath=…' option to replace '$GV_FILE_PATH'

Many of the use cases served by the previously removed $GV_FILE_PATH could be replicated with -Gimagepath=…. Unfortunately there were some that could not. In particular, using a relative path reference beginning with a slash (e.g. image="/foo.png").

This change implements a replacement that is intended to cover all previous use cases. Instead of following the design discussed on the forum,¹ this instead essentially reverts most of 1e45fd83. The new mechanism thus works more or less identically to the old one with the exceptions of (1) being controlled via a command line option instead of an environment variable and (2) not being intertwined with the $SERVER_NAME environment variable.

This mechanism is not intended as a security or sandboxing technology. If you are accepting untrusted input and processing it with Graphviz, it is assumed you are running Graphviz within some other containment technology that restricts its actions.

Gitlab: closes #2396 (closed)
Reported-by: Lannar Dean

¹ https://forum.graphviz.org/t/removal-of-gv-file-path-environment-variable