(Vchar_coding_system_table): Remove this variable.

(Vchar_coding_system_table): Remove this variable.
(Vcoding_system_safe_chars): New variable.
(intersection): Remove this function.
(find_safe_codings): Don't use Vchar_coding_system_table, but try
all codings in SAFE_CODINGS.
(Ffind_coding_systems_region_internal): Adjusted for the change of
find_safe_codings.  Get generic coding systems from
Vcoding_system_safe_chars.
(Fdefine_coding_system_internal): New function.
(syms_of_coding): Defsubr Sdefine_coding_system_internal.
Initialize and staticpro Vcoding_system_safe_chars.