echo(): refactor and rename helper function -> fnwrite()

More often than not, the file to write to in sysfs changes rather than
the value.  This patch changes echo() into a fnwrite(), flipping what
is vsnprintf()'ed, and adds a stupid str() function that converts any
value (float/int/double/uint64_t) to a static string buffer.

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

src/util.h

@@ -45,7 +45,10 @@ extern char *prognm;
 #include <lite/conio.h>
 
 char *progname     (char *arg0);
-int   echo         (char *file, int append, char *fmt, ...);
+
+char *str          (char *fmt, ...);
+int   fnwrite      (char *value, char *fmt, ...);
+
 int   has_utmp     (void);
 int   strtobytes   (char *arg);
 void  do_sleep     (unsigned int sec);