Increase default num boxes

This made it impossible to override the default rules
(which is unlikely to do, though :)).

Also documented behavior wrt. std{in,out,err}.

It turned out that Isolate's signal handling is prone to race
conditions, especially if the machine is under heavy load. Among
other problems:

   o  If multiple SIGINT-like signals were received in a quick
      succession, the "UGH" message appeared.

   o  SIGPIPE could have been caught when writing the meta-file
      to a pipe. This could trigger the same problem as above.

   o  There was a short time window between starting the child
      process and setting up signal handlers, where an interrupt
      signal could have killed the master process and leave the
      child running on its own.

   o  If the master process received a SIGSEGV-like signal,
      it was reported as an error of the sandboxed process
      instead of a proper internal error.

I switched to handling different signals differently:

   o  Interrupt signals are handled synchronously like we already
      did with SIGALRM.

   o  Signals like SIGSEGV are reported as internal errors after
      killing the child process.

   o  SIGPIPE, SIGUSR1, and SIGUSR2 are ignored.

Also, we set up the signal handlers right before calling clone()
and reset them inside the child process, so we should not catch
a signal unprepared.

This significantly decreases the latency of killing the process
after it exceeds its time limit.

Based on a patch by Alexander Crustev.

The previous location in /tmp/box/ was prone to symlink attacks.

Apparently, the switch is ignored for man pages and newer
versions of asciidoc warn on its use.

Inspired by pull request #7 by @bblackham and patch by @austrin.

Closes #4.

It was broken by commit a40942b.

The bug was probably harmless: the cpu/mem sets of the subgroup got
initialized to empty strings by the kernel, which caused it to use
the settings of the parent group anyway.

Added f to short_opts