(__strtol, __strtol_t, __xstrtol):

(__strtol, __strtol_t, __xstrtol):
New macros to specify the
underlying function, its returned type, and our function.  Default to
values suitable for xstrtol.

Include <ctype.h>, since we use its macros.

(ULONG_MAX, LONG_MAX, ISBLANK): Remove: no longer needed.
(ISSPACE): New macro.

(bkm_scale, bkm_scale_by_power, __xstrtol, main): __unsigned long int
-> __strtol_t.

(__xstrtol): Depend on whether __strtol_t is an unsigned type, not on
whether STRING_TO_UNSIGNED is defined.  Skip isspace chars, not
isblank chars, to match strtol.  When returning
LONGINT_INVALID_SUFFIX_CHAR, store the value that we computed into
*val.