daemontools returns an 18-byte status vs. 20 for runit

I tried using this module on daemontools services, but got the error message:

    struct.error: unpack_from requires a buffer of at least 20 bytes

Some investigation revealed that while runit [uses a 20-byte status](https://github.com/pilcrow/runit/blob/master/src/runsvstat.c#L37), daemontools [returns 18 bytes](https://github.com/daemontools/daemontools/blob/master/src/svstat.c#L17). Anyway, this PR is a minimal fix for that issue.