Had to use memset to reset sa_action structure. Without that the signal
handled wasn't installed correctly because of the random data in the
sigaction structure and caused unit tests to fail.

Starting lease manager in the server constructor implied the use of the
default memfile for a short period until the server is configured. The
default memfile configuration used the persistence (writing leases to disk)
which in many cases required root privileges. This caused some unit tests
to fail.

Fixed one typo and clarified in the configuration examples that it is up
to the client to pick the renew and rebind timers if the server doesn't
send option 58 and 59.

The presence of this field caused cppcheck failures.