Allow escaping vars in .env

[markmetcalfe]

@simoncoggins discovered that if you set your TDB_BACKUPS directory to a path that contains special characters or spaces, the tdb command wouldn't work and also you would get warnings printed to the console when running any t command.

This fixes all the t commands to support having escaped characters and variables wrapped in quotation marks in .env so that variables with spaces and special characters can be used.

Implementation note: set -a exports all sourced variables, and then set +a stops them from being exported again.