Don't include <math.h> or <xstrtol.h>; no longer needed.

Don't include <math.h> or <xstrtol.h>; no longer needed.
(isfinite) [!defined isfinite]: New macro.
(separator, terminator): Now points to const.
(first, step, last): Remove.
(usage): Update to match new behavior.
(struct operand, operand): New type.
(scan_arg): Renamed from scan_double_arg, since we no longer use double.
All uses changed.
Compute and return a value of type operand, not double.
(long_double_format): Renamed from valid_format, and now returns a
new format with an "L" added if needed, if the original format was
valid.  Allow %a, %A, %E, %F, and %G formats.
(print_numbers): Take numeric values as args rather than from globals.
Print long double, not double.
(get_width_format): Remove.
(get_default_format): New function.
(main): Implement new way of calculating default format.
Don't worry about locale's representation of the decimal point, since
the arguments are always processed in the C locale.