Way to read password from file

[cafkafk]

We already have passcmd for getting the password from e.g. a password manager (any command invocation). It would be useful for automation for people without a scriptable password manager to have the ability to read a file instead, allowing users to specify it through e.g. various NixOS secrets.

(would also be nice for DBCDK/metamorph@e12d203)