Added a "MinFreeDiskSpace" setting to the root section of sbbsecho.ini

(defaults to 10 megabytes, "10M").
When SBBSecho starts up, if this minimum amount of free space is not
available in any of the following directories, SBBSecho will display an
error and immediately exit:
- data directory (in SCFG)
- logs directory (in SCFG)
- netmail directory (in SCFG)
- outbound directory (from sbbsecho.ini)
- temp directory
- each linked-node's outbox, if configured

If the sysop runs out of disk space for some reason, at least SBBSecho
won't contribute to the problem.