Implement GET DATA/TYPE=TXT.

* automake.mk: Add new tests.

* command/get-data-txt.sh: New test.

* command/get-data-txt-examples.sh: New test.

* command/get-data-txt-importcases.sh: New test.

* data-parser.c: New file.

* data-parser.h: New file.

* data-list.c (struct dls_var_spec): Removed.
(ll_to_dls_var_spec): Removed.
(enum dls_type): Removed.
(struct data_list_pgm): Rename struct data_list_trns.  Remove
pool, specs, type, record_cnt, delims, skip_records, value_cnt
members.  Add new `parser' member.
(cmd_data_list): Use data-parser infrastructure.
(parse_fixed): Ditto.
(parse_free): Ditto.
(dump_fixed_table): Removed.
(dump_free_table): Removed.
(cut_field): Removed.
(read_from_data_list): Removed.
(read_from_data_list_fixed): Removed.
(read_from_data_list_free): Removed.
(read_from_data_list_list): Removed.
(data_list_trns_free): Rename arguments for clarity.
(data_list_trns_proc): Ditto.
(data_list_casereader_read): Removed.
(data_list_casereader_destroy): Removed.
(data_list_casereader_class): Removed.

* get-data.c (cmd_get_data): Support TXT type.
(set_type): New function.
(parse_get_txt): New function.