Java 5 migration

Java 5 migration

* Use foreach loops instead of indexed for loops or iterating while
  loops.
* Use String::contains instead of String::indexOf.
* Use StringBuilder instead of StringBuffer, or a collector when the
  StringBuffer is only used to join a string collection.

This patch also simplifies the return value handling in
FibHelper::doesPrefixBelongToSubnet.

Change-Id: I13c63380e056cdeaa96090c16fee06316db782d9
Signed-off-by: Stephen Kitt <skitt@redhat.com>