Rewrite bgrun/cron

This rewrites bgrun/cron from scratch; the old version had accumulated a
number of hacks over the years.

bgrun/.github/FUNDING.yml

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+github: arp242

bgrun/LICENSE

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+The MIT License (MIT)
+
+Copyright © Martin Tournoij
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to
+deal in the Software without restriction, including without limitation the
+rights to use, copy, modify, merge, publish, distribute, sublicense, and/or
+sell copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in
+all copies or substantial portions of the Software.
+
+The software is provided "as is", without warranty of any kind, express or
+implied, including but not limited to the warranties of merchantability,
+fitness for a particular purpose and noninfringement. In no event shall the
+authors or copyright holders be liable for any claim, damages or other
+liability, whether in an action of contract, tort or otherwise, arising
+from, out of or in connection with the software or the use or other dealings
+in the software.

bgrun/README.md

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+Need to put this in its own repo; just need to finish some things and write
+docs.