Fix bug #11959 with startup warnings on MS-Windows about ../site-lisp.

Fix bug #11959 with startup warnings on MS-Windows about ../site-lisp.

 src/w32.c (init_environment): Change the default values of many
 environment variables in dflt_envvars[] to NULL, to avoid pushing
 them into environment when they were not already defined.  Remove
 the code that deletes site-lisp subdirectories from the default
 value of EMACSLOADPATH, as it is no longer needed.
 (check_windows_init_file): Now external, not static.  Use
 Vload_path as is, without adding anything, as this function is now
 called when Vload_path is already set up.
 src/w32.h (check_windows_init_file): Add prototype.
 src/emacs.c (init_cmdargs) [WINDOWSNT]: When running from the build
 directory, ignore the /*/i386/ tail in Vinvocation_directory, for
 compatibility with Posix platforms.
 (main): Move the call to check_windows_init_file to here from
 w32.c.
 (decode_env_path) [WINDOWSNT]: Expand the %emacs_dir%/ prefix, if
 any, in the DEFALT argument into the root of the Emacs build or
 installation tree, as appropriate.
 src/callproc.c (init_callproc_1): Call decode_env_path instead of
 doing its equivalent by hand.
 (init_callproc): Replace DOS_NT condition with MSDOS, thus letting
 the code that sets Vexec_path run on MS-Windows.
 src/lread.c (init_lread): Add comments to #ifdef's.

 nt/paths.h (PATH_LOADSEARCH, PATH_SITELOADSEARCH, PATH_EXEC)
 (PATH_DATA, PATH_DOC): Replace dummy directory names with
 directories relative to %emacs_dir%.
 (PATH_EXEC): Add lib-src/oo-spd/i386 and lib-src/oo/i386, to cater
 to the use case of running un-installed Emacs.