vgdb must not transmit signals when gdbserver has been ptrace-invoked.

Most of the time, Valgrind masks async signals, and polls for such
signals at regular interval.
There is a very narrow range of code (around client syscall logic)
where such signals are unmasked (as they must be able to interrupt
syscalls).
This is the only range of code where Valgrind is expecting to
receive such a signal.

When gdbserver is ptraced invoked, Valgrind is artificially made
to jump out of this code portion. Signals are not masked.
When ptraceing valgrind, vgdb will get these signals but cannot
transmit them immediately, otherwise they arrive in range
of code where they are not expected (causing internal error
in syscall logic) and/or causing gdbserver syscalls to be
interrupted.

3 solutions to solve that were looked at:
1. have the gdbserver code masking signals "as quickly as possible".
 Easy to implement, but this leaves still a small window
 of code where signals are not masked and would cause a problem.
2. have vgdb setting the SIGMASK of valgrind before invoking
  gdbserver.
  This would be easy to implement, but changing the SIGMASK
  of the ptrace-d process is only available on very recent kernels.
3. have vgdb queuing signals, and only transmitting them once
   gdbserver invocation is finished.
This 3rd solution has been implemented.



git-svn-id: svn://svn.valgrind.org/valgrind/trunk@13896 a5019735-40e9-0310-863c-91ae7b9d1cf9