lib/atoi/: a2i(): Re-implement with a statement expression

lib/atoi/: a2i(): Re-implement with a statement expression

Synopsis
	int a2i(typename T, T *restrict n, QChar *s,
	        QChar **_Nullable restrict endp, int base,
	        T min, T max);

Description
	This macro converts the initial portion of the string pointed to
	by 's' to an integer of base 'base', ensure that the number is
	in the range [min, max], and store it in *n.

	It is similar to NetBSD's strtoi(3) and strtou(3), which
	themselves are similar to strtol(3) and strtoul(3).

Arguments
	T
		The integer type used for the number.
	n
		A pointer to an integer.  The parsed number will be
		stored there.
	s
		See strtol(3).
	endp
		See strtol(3).  A difference with strtol(3) is that this
		macro is const-correct.  If 's' has type 'const char *',
		then 'endp' must have type 'const char **', whereas if
		's' has type 'char *', 'endp' must have type 'char **'.
	base
		See strtol(3).
	min
	max
		See strtoi(3) and strtou(3).

		An important difference with NetBSD's strtou(3) is that
		a2i() (with an unsigned type T) doesn't intepret any
		negative numbers as if they were large positive numbers.
		a2i() respects the limits [min, max] as one would
		intuitively expect.

Return value
	On success, 0 is returned.
	On error, -1 is returned and errno is set to indicate the error.

Errors
	See strtoi(3) and strtou(3).

Examples
	if (a2i(pid_t, &pid, s, &s, 10, 1, _Maxof(pid_t)) == -1)
		goto err;

Signed-off-by: Alejandro Colomar <alx@kernel.org>