Patch #6262. New developers guide and resulting fixes and cleanups.

Patch #6262.  New developers guide and resulting fixes and cleanups.

* automake.mk (src_data_libdata_a_SOURCES): Add new files.

* dict-class.c: New file.
(dict_class_from_id): Move here.
(dict_class_to_name): Move here.

* dict-class.h: New file.
(enum dict_class): Move here.  Change from consecutive integers to
consecutive bits, to make testing for any of multiple values
easier.  Add new DC_ALL constant.

* dictionary.c (struct dictionary): Change `case_limit' from
size_t to casenumber.
(dict_get_vars): Make final argument an enum dict_class.
(dict_get_vars_mutable): Ditto.
(dict_get_case_limit): Change return value to casenumber.
(dict_set_case_limit): Change final argument to a casenumber.
(dict_unset_split_var): Add assertion.  Rephrase slightly.
(dict_set_label): Use xstrndup to simplify.

* format.c (fmt_step_width): AHEX format also needs 2-byte
stepping.
(fmt_set_style): Simplify assertions.

* missing-values.c (mv_add_num_range): Rename mv_add_range.
Simplify implementation.
(mv_has_value): Simplify implementation.
(mv_pop_value): Remove the first value, not the last, to avoid
having GET followed by SAVE reverse the order of missing values.
(mv_peek_value): Rename mv_get_value.  Simplify assertion.
(mv_has_range): Simplify implementation.
(mv_peek_range): Rename mv_get_range.
(can_resize_string): Removed.
(mv_is_resizable): Use value_is_resizable.
(mv_resize): Use value_resize.

* short-names.h (SHORT_NAME_LEN): Move here.

* val-type.h: New file, for definitions related to type and width
of abstract values.  Before, these definitions were mixed among
those related to "union value" and those related to variables.
(macro SYSMIS): Move here.
(macro LOWEST): Move here.
(macro HIGHEST): Move here.
(macro MAX_STRING); Move here.
(enum val_type): New enum with values VAL_NUMERIC and VAL_STRING.
Replaces enum var_type that had values VAR_NUMERIC and VAR_STRING.
All references updated.
(val_type_is_valid): New function.  Replaces var_type_is_valid.
All references updated.
(val_type_from_width): New function.  Replaces
var_type_from_width.  All references updated.

* value-labels.c (val_labs_copy): Renamed val_labs_clone.  All
references updated.
(val_labs_can_set_width): Use value_is_resizable.
(val_labs_add): Simply return false if the value labels set is too
wide, instead of having undefined behavior.
(val_labs_replace): Ditto.
(val_labs_replace): Ditto.
(val_labs_first): Set iterator to null if iteration is complete.
(val_labs_first_sorted): Ditto.
(val_labs_done): Become a no-op if the iterator is null.

* value.c (value_is_resizable): New function.
(value_resize): New function.

* variable.c (var_get_dict_class): New function.

* variable.h (macro LONG_NAME_LEN): Rename VAR_NAME_LEN.  Update
all references.

* not-implemented.texi: Fix @include command so that it works
consistently, by using a file name relative to Makefile.am's
srcdir instead of relative to doc.