The main purpose of this commit is to ensure that exec callbacks are
registered at startup (or config reload), so that they have a chance to
run on the first update interval. Much of the code in exec.cc got
consolidated, making it easier to follow. Several checks for NULL
pointers should eliminate some obscure segfaults I noticed, as well.
During the refactoring, I implemented height and width arguments for
execbar and execgauge. The functionality was already there, but wasn't
being used. All this is accompanied by updates to the man page and
plenty of code comments.
I removed the code for automatic killing of stuck children, because (by admission of the person
who wrote it) it wasn't working. I also removed the $pre_exec variable, because one can use lua
to generate the config on the startup, if one wants to.
Rewrote timed_thread library in C++ using fancy new C++0x features. The
main reason for this is to phase out poor encapsulation and C-style
function pointers.
