improve notify:s6 compatibility

[aanderse]

citing https://skarnet.org/software/s6/notifywhenup.html

• When your daemon is ready, write a newline to this file descriptor.
  • If you like, you may write other data before the newline, just in case it is printed to the terminal. It is not necessary, and it is best to keep that data short. If the line is read by s6-supervise, it will be entirely ignored; only the newline is important.

while testing out eudev support for s6 style readiness notifications i found out that finit is a bit too strict in its implementation

this patch relaxes finit check so it better implements the spec

[troglobit]

Nice work, really appreciate someone else testing out the s6 style notifications!

Gave an inline suggestion for improvement.