RFR(M): 8186072: dll_build_name returns true even if file is missing.

[Lindenmaier, Goetz]

Hi,

dll_build_name builds the proper path to a library given a list of paths seperated by
path_seperator and a library name. It adds in the platform specific endings etc.
It is documented to return whether the file exists, but only does so if a path_seperator
exists in the path.

Especially if the path is empty, it just returns 'true'.
Dll_build_name is usually used before calling dll_load.  If dll_load does not get a full path it searches
in well known unix/windows locations. This is intended in the two cases where dll_build_name
is called with an empty path.
I added a second variant of dll_build_name without the path argument that adds the path
from system property java.lang.path and use that in these two cases.
I changed the original function to actually check file availability in all cases,
and to check . if the path is empty.

Please review this change. I please need a sponsor.
http://cr.openjdk.java.net/~goetz/wr17/8186072-dllBuildName/webrev.01/

Best regards,
  Goetz.