Cygwin: getdtablesize: always return OPEN_MAX_MAX

According to the Linux man page for getdtablesize(3), the latter is
supposed to return "the maximum number of files a process can have
open, one more than the largest possible value for a file descriptor."
The constant OPEN_MAX_MAX is the only limit enforced by Cygwin, so we
now return that.

Previously getdtablesize returned the current size of cygheap->fdtab,
Cygwin's internal file descriptor table.  But this is a dynamically
growing table, and its current size does not reflect an actual limit
on the number of open files.

With this change, gnulib now reports that getdtablesize and
fcntl(F_DUPFD) work on Cygwin.  Packages like GNU tar that use the
corresponding gnulib modules will no longer use gnulib replacements on
Cygwin.

winsup/cygwin/syscalls.cc

@@ -2887,7 +2887,7 @@ setdtablesize (int size)
 extern "C" int
 getdtablesize ()
 {
-  return cygheap->fdtab.size;
+  return OPEN_MAX_MAX;
 }
 
 extern "C" int