switch once env var to fit with others

[deitch]

The env vars for all dump configuration have the following characteristics:

• start with DB_DUMP_
• use the name of the CLI flag after the prefix, converted to upper-case, and with - switched to _

So --begin becomes DB_DUMP_BEGIN, etc.

There is one exception to this, --once which uses the env var RUN_ONCE rather than DB_DUMP_ONCE. This causes a lot of headaches, and libraries that do not handle it correctly, etc.

Since 1.0.0 is not yet GA, we are just switching the env var.