Currently in case of multiple license files they are installed
with their defined abbreviated name.
While that is fine for wellknown license names for "unknown"
once with a file name like License.vendor that is often not
the case.
Try to install the licenses by their actual file name and let the
user define a LICENSE_FILENAME_${lic} to override that if needed.
The latter is helpful in case the multiple licenses are all
called the same but reside in different directories like
foo/License and bar/License.
We try to detect duplicate file names and in that case barf.
While here add a simple target helpful for testing and checking
called list-license.
Sponsored by: The FreeBSD Foundation (partially, tested with wifi-fw)