Fix strace tracing of forked processes when attaching to a process with --pid

[Jon TURNEY]

At the moment, --trace-children (enabled by default) only works when the 
straced process is started by using strace with a command line.

This patch uses the undocumented NtSetInformationProcess(ProcessDebugFlags) 
call to make --trace-children work when attaching to a process with --pid

This patch removes the explicit DebugActiveProcess() on each child process: In 
my testing this was not needed when the process was created using 
CreateProcess() with the DEBUG_PROCESS flag, and failed error 87 when a 
process had been attached to with DebugActiveProcess() and then had the 
DEBUG_ONLY_THIS_PROCESS flag cleared.

In the alternative, the man page should be fixed to mention that tracing
child  processes is only possible when using a command line and not with --pid.

2011-09-12  Jon TURNEY  <jon.turney@dronecode.org.uk>

	* strace.cc (attach_process): Try to turn off DEBUG_ONLY_THIS_PROCESS
	if attaching to a process with the forkdebug flag set.
	(handle_output_debug_string): Apparently we don't need to explicitly
	attach for debugging when a child process starts
	* Makefile.in (strace.exe): Link with ntdll

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