debugger: repeat last command

Regression introduced by commit 9ef9a9d.
debugger: show current line, fix for #6150
test: update require path after file move

Commit 204228b moved a few slow tests to pummel but I forgot to update
the require() path in pummel/test-debugger-repl-break-in-module.  Mea
culpa.
src: raise maximum file descriptor limit

Do a binary search for the maximum RLIMIT_NOFILE.  Works around the
low, low limits on certain high, high-priced devices from Cupertino, CA.
src: notify V8 profiler when we're idle

Inform V8's CPU profiler when we're idle.  The profiler is
sampling-based but not all samples are created equal; mark the wall
clock time spent in epoll_wait() and friends so profiling tools can
filter it out.  The samples still end up in v8.log but with state=IDLE
rather than state=EXTERNAL.
src: only start idle notifier when profiling

The previous commit adds a notifier that tells the V8 profiler when
node.js is idle, i.e. when it's about to start sleeping in the
platform's equivalent of epoll_wait().

This commit adds a heuristic that only starts the notifier when the
V8 profiler is started from the command line.
src: add JS start/stop methods for idle notifier

The previous commit changes the profiler idle notifier so that it only
gets started when a --prof or --prof_lazy argument is specified on the
command line.

This commit adds two internal methods to the process object that allows
one to start and stop the idle notifier programmatically.
src: fix up after botched merge conflict

Mea culpa, I didn't properly resolve a merge conflict in the last two
commits.  The resulting segmentation fault only happened on Linux and
only sometimes.

Fixes #6306.