printf cleanup, to avoid undefined behavior, to add support for

printf cleanup, to avoid undefined behavior, to add support for
formats that Bash supports, and to support wide integers like
Bash does.

(UNSPECIFIED): Remove.  All uses now replaced by
booleans, so that we don't reserve any values for precision or
width (like Bash).
(STRTOX): Use prototype, not K&R-style definition.
(vstrtoimax): Renamed from xstrtol (to avoid confusion with xstrtol
in ../lib), with type change to intmax_t.
All uses changed.
(vstrtoumax): Renamed from xstrtoul, with type change to uintmax_t.
All uses changed.
(vstrtod): Renamed from xstrtod.  All uses changed.
(print_direc): Use boolean arg instead of special value to indicate
a missing precision or width.  LENGTH no longer includes
length modifiers or conversion character.  New arg CONVERSION
now specifies conversion character.
Use intmax_t-width formatting for integers (like Bash).
Add support for C99 %a, %A, %F (like Bash).
Add support for field width with %c (POSIX requires this).
Add a FIXME for lack of support for field width and precision
for %b.
Add support for '\'', '0' flags.
Check for invalid combinations of flags, field width, precision,
and conversion, to prevent use of undefined behavior.
Allow multiple length modifiers, for formats like "%lld" (like Bash).
Add support for C99 'j', 't', 'z' length modifiers (like Bash).
In error message, output entire invalid conversion specification,
instead of merely outputting % followed by the conversion char.