Signed integers can be checked more easily for integer overflow.
* lib/dfa.c (position, struct lexer_state, struct parser_state)
(struct dfa, mbs_to_wchar, fetch_wc, parse_bracket_exp)
(struct lexptr, lex, addtok_mb, add_utf8_anychar, atom)
(nsubtoks, copytoks, closure, alloc_position_set, delete)
(replace, state_index, epsclosure, charclass_context)
(state_separate_contexts, merge_nfa_state, dfaoptimize)
(dfaanalyze, build_state, dfaexec_main, dfa_supported)
(maybe_disable_superset_dfa, dfassbuild, dfafree, enlist)
(comsubs, inboth, allocmust):
Prefer a signed to an unsigned integer when calculating indexes,
unless the integer is part of the external API (a bigger deal,
and to be done later).