Add whichp() predicate function to which()

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

README.md

@@ -288,10 +288,12 @@ Generic Functions
   optional listing of file and directory permissions if `show_perms` is
   set.  The `path` argument should be a directory.
 
-- `which(cmd)`
+- `which(cmd)`, `whichp(cmd)`
 
   C implementation of UNIX which(1).  Returns a malloc'ed string with the
   full path to `cmd` on success, otherwise `NULL`.
+  
+  `whichp()` is a predicate function, returns `TRUE` or `FALSE`.
 
 
 GNU Functions