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cgraph: rewrite 'aglasterr' to operate in-memory

Matthew Fernandez requested to merge smattr/graphviz:smattr/69371535-f225-450f-be33-297f6cabd8bb into main

aglasterr was using a temporary file to stash the last warning/error message. There were numerous downsides to this:

  1. Warnings/errors would result in unnecessary disk I/O. This puts extra wear on storage devices, decreases efficiency, and puts the operating system’s I/O subsystem on the path to Graphviz printing a warning/error.

  2. During a run of Graphviz, the temporary file is never truncated or overwritten. Large warning/error output would gradually fill the user’s disk.

  3. If the current environment has no temporary directory (or an entirely read-only file system) or the maximum number of temporary files has been exceeded, this mechanism would fail.

This change avoids the above issues by maintaining the last warning/error message in an in-memory buffer.

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