Request to direct SCR log messages to stderr or a custom file descriptor

[adammoody]

When running with SCR_DEBUG=1, SCR prints log messages to stdout. It would be useful at times to direct those messages to other files like stderr or perhaps a user-provided file descriptor. This would enable users to more easily filter SCR messages from their job logs.

The related code in SCR is in scr_err_mpi.c and scr_err_serial.c, e.g.:

https://github.com/LLNL/scr/blob/48328f6c1a0fdb5a3187bd4c38d0caac587442a8/src/scr_err_mpi.c#L52-L63

There are a few other places where SCR calls print functions of other components, like:
https://github.com/LLNL/scr/blob/48328f6c1a0fdb5a3187bd4c38d0caac587442a8/src/scr.c#L1278-L1289