Add whichp() predicate function to which()

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

lite.h

@@ -86,6 +86,7 @@ void    progress       (int percent, int max_width);
 void    progress_simple(int percent);
 
 char   *which      (const char *cmd);
+int     whichp     (const char *cmd);
 
 #ifndef touch
 #include <sys/stat.h>		/* utimensat() */