Use dladdr/GetModuleHandleEx() to find plugin dir

Using hard-coded paths to find plugins is nice for properly installed
builds, but it interferes with testing when Heimdal is also installed.

For now we hardcode $ORIGIN/../lib/plugin/krb5 (need to fix this for
Windows?) and we eval $ORIGIN by using dladdr() (Linux, Solaris) or
GetModuleHandleEx() (Win32) to find the path to libkrb5 whose dirname to
use as $ORIGIN.