Rename the input_spec pragma to input_mode_spec ...

... as requested by Julien.

compiler/prog_item_pragma.m:
    Rename the pragma's internal name.

compiler/parse_pragma.m:
    Update the code reading in the pragma ...

compiler/parse_tree_out_pragma.m:
    ... and the code writing out the pragma.

compiler/add_pragma_decl.m:
compiler/convert_parse_tree.m:
compiler/equiv_type_parse_tree.m:
compiler/item_util.m:
compiler/make_hlds_passes.m:
compiler/make_hlds_separate_items.m:
compiler/module_qual.qualify_items.m:
compiler/parse_tree_out.m:
compiler/parse_type_name.m:
compiler/prog_item_stats.m:
compiler/prog_parse_tree.m:
compiler/recompilation.version.m:
tests/hard_coded/test_input_spec.m:
tests/invalid/bad_input_spec.{m,err_exp}:
    Conform to the changes above.