Handling Cygwin DLLs in new foreign-library.scm

[Mike Gran]

Hello Guile-
By convention, DLLs in cygwin start with 'cyg' instead of 'lib' to avoid name 
clashing.
In an autotools build like Guile, dlls are build as 'cyg'
Now that we've dumped libltdl, we don't automatically search for both 'lib' and 
'cyg' on cygwin.
Should we emulate this in foreign-library.scm to mimic old behavior?
Or rather should we expect a user to add special cases for cygwin with 
load-library.

Regards,
Michael