bug#316: XFLD fonts broken on w32 since around font-backend merge

[Jason Rumney]

Since sometime around the font backend merge, fonts specified as XFLD no 
longer work on Windows.

 emacs -Q -fn "-*-DejaVu Sans Mono-normal-r-*-*-13-*-*-*-c-*-iso8859-1"
results in Emacs not starting, and the following message on stderr:
 Font `-*-DejaVu Sans Mono-normal-r-*-*-13-*-*-*-c-*-iso8859-1' is not 
defined.

Emacs now seems to only recognize that font if the slant is specified as 
"normal" rather than "r", and the spacing as "m" (previous versions of 
Emacs on Windows only used "c" and "p" here, but up until the merge of 
font-backend code, Emacs seemed to be happy using a font with 
FONT_SPACING_MONO when "c" was specified).