alternative password supply

feed password to weave via env var, possibly via a file …
The latter is the most secure route since it keeps the password
completely out of any command lines, and the permissions on the file
can be very restrictive, e.g. 0400.

It would be somewhat nicer if the password file contained just
    <password>
rather than
    WEAVE_PASSWORD=<password>
but then we'd have to transit the password via a temporary file with
0600 permissions.

Closes #276.