Add a `runtime:db-path` var

[krader1961]

A recent problem by a user illustrated the need for a way to determine the path to the interactive database file without the user having to manually apply the rules for determining the path. A $runtime:db-path var would make it unambiguous what file Elvish is using.

[krader1961]

FWIW, I was going to implement this feature but decided not to because I still hope that issue #1222 is resolved. Thus eliminating the need for an Elvish daemon by replacing its reliance on BoltDB with a simpler flat file mechanism that does not require a daemon to coordinate access to the interactive history.

[krader1961]

I'm closing this because I hope my work to replace the Elvish daemon and dependency on the BoltDB project is merged in the not to distant future which will make this issue moot.