Permit XLFD names to exceed 255 characters

Permit XLFD names to exceed 255 characters

* doc/lispref/display.texi (Low-Level Font)
<font-xlfd-name>: Document new argument `long-xlfds'.

* etc/NEWS: Mention removal of XLFD length restrictions.

* src/font.c (font_build_object): Dynamically allocate XLFD,
permitting them to surpass 255 characters in length.
(font_parse_xlfd_1): Cease rejecting XLFDs more than 255
characters in length.
(font_dynamic_unparse_xlfd): New function.  Like
font_unparse_xlfd, but allocate the XLFD dynamically.
(font_delete_unmatched): Dynamically allocate XLFD if necessary.
(Ffont_xlfd_name): New arg LONG_XLFDs.  If t, return a
dynamically allocated XLFD.  All callers changed.

* src/font.h: Update prototypes.

* src/fontset.c (Fnew_fontset): Dynamically allocate XLFD when
establishing fontset name.