implement strftime padding specifier extensions

notes added by maintainer:

the '-' specifier allows default padding to be suppressed, and '_'
allows padding with spaces instead of the default (zeros).

these extensions seem to be included in several other implementations
including FreeBSD and derivatives, and Solaris. while portable
software should not depend on them, time format strings are often
exposed to the user for configurable time display. reportedly some
python programs also use and depend on them.

src/time/strftime.c

@@ -48,12 +48,12 @@ static int week_num(const struct tm *tm)
 const char *__tm_to_tzname(const struct tm *);
 size_t __strftime_l(char *restrict, size_t, const char *restrict, const struct tm *restrict, locale_t);
 
-const char *__strftime_fmt_1(char (*s)[100], size_t *l, int f, const struct tm *tm, locale_t loc)
+const char *__strftime_fmt_1(char (*s)[100], size_t *l, int f, const struct tm *tm, locale_t loc, int pad)
 {
 	nl_item item;
 	long long val;
 	const char *fmt = "-";
-	int width = 2;
+	int width = 2, def_pad = '0';
 
 	switch (f) {
 	case 'a':
@@ -79,15 +79,14 @@ const char *__strftime_fmt_1(char (*s)[100], size_t *l, int f, const struct tm *
 	case 'C':
 		val = (1900LL+tm->tm_year) / 100;
 		goto number;
+	case 'e':
+		def_pad = '_';
 	case 'd':
 		val = tm->tm_mday;
 		goto number;
 	case 'D':
 		fmt = "%m/%d/%y";
 		goto recu_strftime;
-	case 'e':
-		*l = snprintf(*s, sizeof *s, "%2d", tm->tm_mday);
-		return *s;
 	case 'F':
 		fmt = "%Y-%m-%d";
 		goto recu_strftime;
@@ -200,7 +199,12 @@ const char *__strftime_fmt_1(char (*s)[100], size_t *l, int f, const struct tm *
 		return 0;
 	}
 number:
-	*l = snprintf(*s, sizeof *s, "%0*lld", width, val);
+	switch (pad ? pad : def_pad) {
+	case '-': *l = snprintf(*s, sizeof *s, "%lld", val); break;
+	case '_': *l = snprintf(*s, sizeof *s, "%*lld", width, val); break;
+	case '0':
+	default:  *l = snprintf(*s, sizeof *s, "%0*lld", width, val); break;
+	}
 	return *s;
 nl_strcat:
 	fmt = __nl_langinfo_l(item, loc);
@@ -221,7 +225,7 @@ size_t __strftime_l(char *restrict s, size_t n, const char *restrict f, const st
 	char buf[100];
 	char *p;
 	const char *t;
-	int plus;
+	int pad, plus;
 	unsigned long width;
 	for (l=0; l<n; f++) {
 		if (!*f) {
@@ -233,6 +237,8 @@ size_t __strftime_l(char *restrict s, size_t n, const char *restrict f, const st
 			continue;
 		}
 		f++;
+		pad = 0;
+		if (*f == '-' || *f == '_' || *f == '0') pad = *f++;
 		if ((plus = (*f == '+'))) f++;
 		width = strtoul(f, &p, 10);
 		if (*p == 'C' || *p == 'F' || *p == 'G' || *p == 'Y') {
@@ -242,7 +248,7 @@ size_t __strftime_l(char *restrict s, size_t n, const char *restrict f, const st
 		}
 		f = p;
 		if (*f == 'E' || *f == 'O') f++;
-		t = __strftime_fmt_1(&buf, &k, *f, tm, loc);
+		t = __strftime_fmt_1(&buf, &k, *f, tm, loc, pad);
 		if (!t) break;
 		if (width) {
 			for (; *t=='+' || *t=='-' || (*t=='0'&&t[1]); t++, k--);