Allow custom global sincedb target directory

This request adds a SINCEDB_DIR environment variable to isolate the sincedb target location from the HOME variable.  I recognize that HOME is only used for SINCEDB_DIR purposes currently, but this will shield it from any future use of HOME.  
When SINCEDB_DIR is present, it will take precedence, but if it is missing, then no behavior will change.  I've included a new test for input/file to verify the functionality.  I did not update any documentation as I could not find anyplace that made sense to document this.

Thanks,
Tejay