extend fractional duration support to "top -d N.N" and "timeout"

function                                             old     new   delta
parse_duration_str                                     -     168    +168
sleep_for_duration                                     -     157    +157
top_main                                             885     928     +43
timeout_main                                         269     312     +43
handle_input                                         571     614     +43
duration_suffixes                                      -      40     +40
sfx                                                   40       -     -40
sleep_main                                           364      79    -285
------------------------------------------------------------------------------
(add/remove: 4/1 grow/shrink: 3/1 up/down: 494/-325)          Total: 169 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

libbb/duration.c

@@ -0,0 +1,76 @@
+/* vi: set sw=4 ts=4: */
+/*
+ * Utility routines.
+ *
+ * Copyright (C) 2018 Denys Vlasenko
+ *
+ * Licensed under GPLv2, see file LICENSE in this source tree.
+ */
+//config:config FLOAT_DURATION
+//config:	bool "Enable fractional duration arguments"
+//config:	default y
+//config:	help
+//config:	Allow sleep N.NNN, top -d N.NNN etc.
+
+//kbuild:lib-$(CONFIG_SLEEP)   += duration.o
+//kbuild:lib-$(CONFIG_TOP)     += duration.o
+//kbuild:lib-$(CONFIG_TIMEOUT) += duration.o
+
+#include "libbb.h"
+
+static const struct suffix_mult duration_suffixes[] = {
+	{ "s", 1 },
+	{ "m", 60 },
+	{ "h", 60*60 },
+	{ "d", 24*60*60 },
+	{ "", 0 }
+};
+
+#if ENABLE_FLOAT_DURATION
+duration_t FAST_FUNC parse_duration_str(char *str)
+{
+	duration_t duration;
+
+	if (strchr(str, '.')) {
+		double d;
+		char *pp;
+		int len = strspn(str, "0123456789.");
+		char sv = str[len];
+		str[len] = '\0';
+		errno = 0;
+		d = strtod(str, &pp);
+		if (errno || *pp)
+			bb_show_usage();
+		str += len;
+		*str-- = sv;
+		sv = *str;
+		*str = '1';
+		duration = d * xatoul_sfx(str, duration_suffixes);
+		*str = sv;
+	} else {
+		duration = xatoul_sfx(str, duration_suffixes);
+	}
+
+	return duration;
+}
+void FAST_FUNC sleep_for_duration(duration_t duration)
+{
+	struct timespec ts;
+
+	ts.tv_sec = MAXINT(typeof(ts.tv_sec));
+	ts.tv_nsec = 0;
+	if (duration >= 0 && duration < ts.tv_sec) {
+		ts.tv_sec = duration;
+		ts.tv_nsec = (duration - ts.tv_sec) * 1000000000;
+	}
+	do {
+		errno = 0;
+		nanosleep(&ts, &ts);
+	} while (errno == EINTR);
+}
+#else
+duration_t FAST_FUNC parse_duration_str(char *str)
+{
+	return xatou_range_sfx(*argv, 0, UINT_MAX, duration_suffixes);
+}
+#endif