File-handling cleanup in w32image.c

File-handling cleanup in w32image.c

* src/w32image.c (w32_load_image): Encode the image file name and
convert it via 'map_w32_filename'.  No need to do anything special
when 'w32_unicode_filenames' is zero, since file names are in
UTF-8 internally, and this code will never run on Windows 9X.
* src/w32.h (map_w32_filename): Add prototype; removed prototypes
from all *.c files.