Add random offset 0..3000 to port search loop

Also add more comments
Rename PortUtil.available() to isAvailable()
PortUtil.getAvailablePort() should throw an exception if no port is available

Returning -1 is C, not Java.
PortUtil: first unprivileged port is 1024, not 1025
Fix incorrect Javadoc and minor white space issues
Remove randomness from search offset, make it explicit and documented

A mandatory randomness is kinda unreproducible. It is better to leave it up to
the caller if she wants to have a random offset or not. Users who really want to
check all ports >= 1024 for availability might get the response that none are
available just because a few thousands of them have been randomly skipped.
Clean-code some local variable names
Fix calculation of MAX_PORT_OFFSET and offset usage in getAvailablePort(int)