lib: fix notification by reading one byte at a time from Unix socket

lib: fix notification by reading one byte at a time from Unix socket

Internally, several bytes will be read at once and the notification
should be detected earlier. This fixes a regression introduced in
5aeda65433fd41b7bb67f908d8c4741f045ce508 because we relied on datagram
boundary semantics. This was not important when pulling information
but we still used this semantic when pushing information. Closes: #43.