Reduce size of wrap.c

• pv-adverb: Avoid jumping past initialization

  When we add proper error-checking for determining the path to /proc/self/exe, which will be done in a subsequent commit, it will involve a 'goto out' that jumps past initialization of this variable.

• pv-adverb, pv-wrap: Factor out _srt_find_executable_dir

  This reduces the size of wrap.c, which is (still) uncomfortably large, and cannot easily be unit-tested because it contains main().

  This incidentally also adds error-checking to the use in pv-adverb, which previously did not check for errors when reading /proc/self/exe.

• pv-wrap: Factor out pv_wrap_maybe_load_nvidia_modules

  This reduces the size of wrap.c, which is too big.