Currently, top(1) interactive mode features a 'u' command to filter the view so
that only processes owned by a given username are displayed.
This changeset enhances the filtering to support the two additional formats
"+username" and "-username". This allows to display processes owned by a set of
usernames. Additionally to resetting the default (pass-all) filter with a
single "+", this changeset allows for the same behaviour with a single "-".
The maxium allowed number of usernames is currently hard-coded to 8 with a
macro. I am open to i) change the number or ii) make it dynamic at runtime, if
any is desired.