Rework how screen size is tracked

The screen size is fetched after a SIGWINCH is delivered. The current
implementation has two issues:

* It calls ioctl() from the SIGWINCH signal handler, despite ioctl() not
  being a function that is known to be safe to call.
* It's not thread-safe.

Signals can be delivered on arbitrary threads, so we don't know if it's
actually safe to be modifying the cached winsize in response to a
signal. It's also plausible that the winsize may be requested from a
background thread.

To solve the first issue, we twiddle a volatile boolean flag in the
signal handler and defer the ioctl() call until we actually request the
screen size.

To solve the second issue, we introduce a pthread rwlock around the
cached winsize. A rwlock is used because it can be expected that there
are likely to be far more window size reads than window size writes. If
we were using C++11 we could probably get away with atomics, but since
we don't have that (or boost), a rwlock should suffice.

Fixes #1613.

function.cpp

@@ -67,9 +67,6 @@ void function_autoload_t::command_removed(const wcstring &cmd)
     function_remove_ignore_autoload(cmd);
 }
 
-/* Helper macro for vomiting */
-#define VOMIT_ON_FAILURE(a) do { if (0 != (a)) { int err = errno; fprintf(stderr, "%s failed on line %d in file %s: %d (%s)\n", #a, __LINE__, __FILE__, err, strerror(err)); abort(); }} while (0)
-
 /**
    Kludgy flag set by the load function in order to tell function_add
    that the function being defined is autoloaded. There should be a