Try to simplify the initialization of Vload_path

Try to simplify the initialization of Vload_path

* src/lread.c (load_path_check): New function, split from init_lread.
(init_lread): Reorganize.  Motivation:
If EMACSLOADPATH is set, check/warn about that rather than the
defaults, which we are not going to use.  Hence we can remove
the turn_off_warning and WINDOWSNT || HAVE_NS tests.
Don't warn if site-lisp directories are missing.
If not installed, start from a blank load-path, since
PATH_LOADSEARCH refers to the eventual installation directories.