Remove leftover test code for Windows

- Removed all code that only runs under CYGWIN, and made all
  code that doesn't run under CYGWIN non-optional.
- Removed the $TP variable which was used to add optional
  trailing dots to filenames; they're no longer optional.
- Removed references to pssuspend and dos2unix.
- No need to use environment variables for diff and kill.
- Removed uses of "tr -d '\r'"; this was a workaround for
  a cygwin regex bug that is no longer needed.

bin/tests/system/keepalive/tests.sh

@@ -71,7 +71,7 @@ echo_i "checking re-configured value ($n)"
 ret=0
 n=`expr $n + 1`
 $RNDCCMD tcp-timeouts 300 300 300 200 > output
-$DIFF -b output expected || ret=1
+diff -b output expected || ret=1
 $DIG $DIGOPTS +vc +keepalive foo.example @10.53.0.2 > dig.out.test$n
 grep "; TCP KEEPALIVE: 20.0 secs" dig.out.test$n > /dev/null || ret=1
 if [ $ret != 0 ]; then echo_i "failed"; fi