libutil: add function to concatenate a format string in place

Add function util_concatf() that appends the result of a format string
expansion to the end of an existing string while taking care of the
required memory allocations.

Usage example:

  char *str = NULL;

  util_concatf(&str, "list:");
  for (int i = 1; i <= 3; i++)
    util_concatf(&str, "%spart%d", (i > 1 ? "," : ""), i);
  printf("%s\n", str); /* list:part1,part2,part3 */

Reviewed-by: Jan Höppner <hoeppner@linux.ibm.com>
Signed-off-by: Peter Oberparleiter <oberpar@linux.ibm.com>
Signed-off-by: Steffen Eiden <seiden@linux.ibm.com>

include/lib/util_libc.h

@@ -127,6 +127,7 @@ do {							\
 int __util_vsprintf(const char *func, const char *file, int line,
 		    char *str, const char *fmt, va_list ap);
 char *util_strcat_realloc(char *str1, const char *str2);
+void util_concatf(char **str1, const char *fmt, ...);
 void util_str_toupper(char *str);
 
 char *util_strstrip(char *s);