readcfg: read from a passwordfile for freeipmi mode

[jason-ni-0]

Adds a passwordfile config option to read passwords from external files instead of storing them inline, mostly motivated by security reasons. If both password and passwordfile are defined, passwordfile takes precedence with a warning logged.

Example in cf file:

console myserver {
    type ipmi;
    host host1;
    username admin;
    passwordfile /path/to/password;
    master localhost;
}

Let me know if an alternative way to do this is exists/preferred, thanks for looking.