Add --grace-period flag

[mdavidsaver]

Add --grace-period flag. Combines with --killsig to allow time for a graceful child shutdown during procServ shutdown.

By default, no change. If a killsig other than SIGKILL is selected, along with a non-zero grace period, then when procServ receives SIGTERM:

1. Send killsig to child
2. wait up to grace-period
3. If child remains, then issue SIGKILL
4. procServ exit

There are some whitespace changes in a separate commit.

[mdavidsaver]

The basic logic is to add another variable to main() (stopAt) which is set to a non-zero value when a graceful shutdown is in-progress.

Documentation update waiting on #68.

[mdavidsaver]

cf. epics-base/epics-base#670

[ralphlange]

Looks good to me.

[mdavidsaver]

Superseded by #75.