use clock() as source for SRV randomness

use clock() as source for SRV randomness

Initializing a random number generator with the time since epoch could
be good enough, but reaches its limits in test code as the 100
iterations might very well happen in the same second and hence the seed
number is always the same… clock() has a way lower resolution so it
changes more often and not unimportant: If many users start the update
at the same time it isn't to unlikely the SRV record will be ordered in
the same second choosing the same for them all, but it seems less likely
that the exact same clock() time has passed for them.

And if I have to touch this, lets change a few other things as well to
make me and/or compilers a bit happier (clang complained about the usage
of a GNU extension in the testcase for example).