unixsock utils: Retry reading from socket if the call was interrupted.
sysdb: Only strip newlines from the command buffer.

Keep whitespace in order to keep the users formatting and keep semicolons to
ensure correct parsing (avoid possible future ambiquity).
sysdbd.conf(5), sysdbd.conf: Documented the 'Listen' option.

And added a sample configuration.
sysdb: Added a query to the history after executing it.

Mimic zsh's histignorespace option and ignore commands when the first
character is a space.