* strace.cc (forkdebug): Make true by default.

(attach_process): Use window pid if cygwin pid isn't available (yet).
(create_child): Use either DEBUG_ONLY_THIS_PROCESS or DEBUG_PROCESS,
exclusively.  (Suggested by Conrad.Scott@dsl.pipex.com)

winsup/utils/utils.sgml

@@ -645,7 +645,7 @@ Usage strace [options] program [args...]
 
   -b, --buffer-size=SIZE       set size of output file buffer
   -d, --no-delta               don't display the delta-t microsecond timestamp
-  -f, --trace-children         also trace forked child processes
+  -f, --trace-children         trace child processes (toggle -- default is "true")
   -h, --help                   display help info
   -m, --mask=MASK              set message filter mask
   -n, --crack-error-numbers    output descriptive text instead of error