Changes:

	- partly fix the multitracing problems.

	  ugdb still can't work with ptrace, ptrace-utrace.c needs
	  changes. But at least multiple udgb tracers can coexist,
	  more or less.

	  But of course they can confuse each other anyway, no matter
	  what ugdb does.

	- implement memory writes ($M).

	- refactor memory reads to avoid the "Too late to report the
	  error" case.

But, Jan. Implementing the memory writes does not mean breakpoints
automatically start to work!

Yes, gdb writes cc, and yes the tracee reports SIGTRAP. But after
that "continue" does nothing except "$c", and the tracee naturally
gets SIGILL. I expected that, since ugdb doesn't even know the code
was changed, gdb should write the original byte back before continue,
but this doesn't happen.

Tried to understand how this works with gdbserver, but failed so far.
Will continue, but any hint is very much appreciated ;)

Oleg.