grdc(6): fix timekeeping for user-supplied value n
grdc(6): fix timekeeping for user-supplied value n

- Keep timespec 'now' and 'delay' separate to avoid confusion
- Increase user-supplied n to run _at least_ n seconds, not max n

PR:            bin/149130 (based on)
Submitted by:  Andy Farkas
MFC after:     2 weeks

jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase