Add _MAXCOMLEN to syslimits, it is 24, and includes the NUL. Following

Add _MAXCOMLEN to syslimits, it is 24, and includes the NUL.  Following
commits will change sys/param.h MAXCOMLEN, sys/sysctl.h KI_MAXCOMLEN,
sys/acct.h ac_comm[], dev/ic/dtvar.h DTMAXCOMLEN to use this, and also
adapt most to the included NUL correctly.  This will allow us to remove
sys/param.h include from userland base.
sys/syslimits.h is chosen as the best place for this define because it
has very small namespace which is already pulled in often.
ok millert