* coding.c (QCcategory): New variable.

* coding.c (QCcategory): New variable.
(syms_of_coding): Defsym it.  Set all elements of
Vcoding_category_table and their symbol values.
(Fset_coding_system_priority): Doc fix.  Update symbol qvalues of
coding-category-XXX, and coding-category-list.
(Fdefine_coding_system_internal): Add category in the plist.