Avoid overwriting buffers with long pathnames

Avoid overwriting buffers with long pathnames

Reported, with initial patch, by Gisle Vanem <gvanem@online.no>

* src/main.c (find_and_set_default_shell) [W32]: Pass search_token
directly to w32ify: no need to make a copy first.  When we need to
construct a path, use snprintf() to be sure we don't overwrite
the locally-allocated buffer.
* src/w32/pathstuff.c (w32ify) [W32]: Use the malloc version of
_fullpath(), followed by strncpy(), to avoid overwriting buffers.