(GET_NICE_VALUE): Renamed from GET_PRIORITY.

(GET_NICE_VALUE): Renamed from GET_PRIORITY.
All uses changed.
(NZERO): New macro, if system doesn't define it already.
(usage): Distinguish priorities from nice values.
Don't assume NZERO is 20.
(main): Use bool instead of int where appropriate.
If user specifies an adjustment out of range, always truncate it
to an inrange value instead of sometimes giving an error message
and sometimes not.
Do not assume that -1 is an error return from "nice" or
"getpriority", as it might be the current nice value minus NZERO.
If nice/setpriority fails with errno == EPERM, go ahead and run
the command anyway; POSIX requires this.