On Cygwin, use /proc file system instead of win32 API.

On Cygwin, use /proc file system instead of win32 API.

* lib/relocatable.c: On Cygwin, use file names from /proc, rather than
Win32 file names.
(DllMain): Simplify by removing Cygwin specific code.
(find_shared_library_fullname): Use Linux specific implementation also
for Cygwin.
(get_shared_library_fullname): Update accordingly.
* lib/progreloc.c: On Cygwin, use file names from /proc, rather than
Win32 file names.
(find_executable): On Cygwin, use /proc, like on Linux. Remove previous
Cygwin specific code.