MFC r343222-r343223, r343338

r343222:
Fix crash in systat(4) when certain commands are called without arguments

Add check for missing arguments to dsmatchselect and dsselect

PR: 219689
Submitted by: Marko Turk <mt@markoturk.info>

r343223:
Fix inconsistency in return values introduced by r343222

Consistently return 1 or the case of missing arguments in both functions

PR: 219689
X-MFC-With: 343222

r343338:
Fix systat's :only command parser for the multiple arguments case

According to systat(1) :only option is supposed to accept multiple drives
but the parser for its arguments stops after first entry. Fix the parser
logic to accept multiple drives.

PR: 59220
Reported by: Andy Farkas <andyf@speednet.com.au>