rebased tail of dfa/MBS_SUPPORT-cleanup series

[Jim Meyering]

This is the rebased tail of the dfa/MBS_SUPPORT-cleanup series.
It starts with the change that adds these lines to mbsupport.h:

#if ! MBS_SUPPORT
# undef MB_CUR_MAX
# define MB_CUR_MAX 1
#endif

The remaining patches take advantage of that information
to simplify many expressions involving those two symbols
and related ones like d->mb_cur_max:

[1/5] maint: ensure that MB_CUR_MAX is defined even when
[2/5] build: fix compilation failure when MBS_SUPPORT is 0
[3/5] maint: dfa: avoid in-function "#if MBS_SUPPORT" tests
[4/5] maint: dfa: simplify several expressions
[5/5] maint: dfa: simplify multi-byte-related conditionals