maint: distinguish EOVERFLOW vs ERANGE better

maint: distinguish EOVERFLOW vs ERANGE better

Also, prepare for allowing some arguments to overflow
without that being an error.
* gl/lib/xdectoint.c: Do not include stddef.h,
since we no longer use ‘unreachable’.
(xnumtoimax, xnumtoumax, __xnumtoint):
New arg FLAGS.  All callers changed.
Stop using __xdectoint_signed.  All definers removed.
* gl/lib/xdectoint.h (XTOINT_MIN_QUIET, XTOINT_MAX_QUIET)
(XTOINT_MIN_RANGE, XTOINT_MAX_RANGE): New flag constants.
* src/fmt.c (main):
* src/fold.c (main):
* src/nl.c (main):
* src/pr.c (getoptnum):
* src/split.c (main):
Use XTOINT_MIN_RANGE and XTOINT_MAX_RANGE if appropriate.
* src/pr.c (getoptnum): Return int rather than returning void
and storing through int *.
* src/stty.c (apply_settings):
Use ckd_add to check for overflow instead of doing it by hand.
(integer_arg): Accept and return uintmax_t, not unsigned long.