Synchronous breakpoints make sure to keep a ptrace-stepped thread
synchronized with its siblings from the same process running in the
background, as follows:

- as soon as a ptracer (e.g. gdb) regains control over a thread which
  just hit a breakpoint or received SIGINT, sibling threads from the
  same process which run out-of-band are immediately frozen.

- all sibling threads which have been frozen are set to wait on a
  common barrier before they can be released. Such release happens
  once all of them have joined the barrier in out-of-band context,
  after the (single-)stepped thread resumed.

Signed-off-by: Philippe Gerum <rpm@xenomai.org>