Based on an idea suggested by Job Snijders <job@sobornost.net>,
a -s command option has been implemented, with great deal of inspiration
from OpenBSD.
When used, it allows only a single instance of given command
to run at once, allowing the next one to run only after the previous
one finishes.
For this to happen, job queue has been reworked to use queue instead
of a simple linked list.