Be a bit more systematic about using 'abort'.

Be a bit more systematic about using 'abort'.
* lib/abitset.c (abitset_test): Remove ATTRIBUTE_UNUSED; not needed.
* lib/bitset.c (bitset_bytes, bitset_init, bitset_op4_cmp):
Put 'default: abort ();' before some other case, to satisfy older
pedantic compilers.
* lib/bitset_stats.c (bitset_stats_init): Likewise.
* lib/ebitset.c (ebitset_elt_find, ebitset_op3_cmp): Likewise.
* lib/lbitset.c (lbitset_elt_find, lbitset_op3_cmp): Likewise.
* src/conflicts.c (resolve_sr_conflict): Likewise.
* src/vcg.c (get_color_str, get_textmode_str, get_shape_str):
(get_decision_str, get_orientation_str, get_node_alignment_str):
(get_arrow_mode_str, get_crossing_type_str, get_view_str):
(get_linestyle_str, get_arrowstyle_str): Likewise.
* src/conflicts.c (resolve_sr_conflict):
Use a default case rather than one for the one remaining enum
value, to catch invalid enum values as well.
* src/lalr.c (set_goto_map, map_goto):
Prefer "assert (FOO);" to "if (!FOO) abort ();".
* src/nullable.c (nullable_compute, token_definitions_output):
Likewise.
* src/reader.c (packgram, reader): Likewise.
* src/state.c (transitions_to, state_new, state_reduction_find):
Likewise.
* src/symtab.c (symbol_user_token_number_set, symbol_make_alias):
(symbol_pack): Likewise.
* src/tables.c (conflict_row, pack_vector): Likewise.
* src/scan-skel.l (QPUTS): Remove unnecessary parens.
(BASE_QPUTS, "@output ".*\n): Remove unnecessary asserts.
* src/system.h: Don't include <assert.h>.
(assert): New macro.