find_val_for_string() is only used in prefs.c so remove the prototype

find_val_for_string() is only used in prefs.c so remove the prototype
and make it static.

When reading enum preferences, use the current value of the preference as the
default value of the preference instead of a hard-coded 1.  This way if we
couldn't find the appropriate value we'll don't end up changing the preference
to something that may or may not make sense.

git-svn-id: http://anonsvn.wireshark.org/wireshark/trunk@32950 f5534014-38df-0310-8fa8-9805f1628bb7