Traverse a directory tree in C using a function pointer.
The traversewfp() function is the same for all examples below.
What changes between examples is the set of function pointers passed to traversewfp().

Usage:

    make clean all run

Examples:

• rm-rf: equivalent to 'rm -rf'
• ls-r: similar to 'ls -R'
• find-type: similar to 'find . -type -f -o -type -d'
• find-regex: similar to 'find . -name "*html" -o -name "*txt"'
• count-files: similar to 'find . | wc -l'

Set of function pointers (see source code examples for usage):

    action_status_t on_opendir_err;                    /* What to do on opendir() error? */
    action_status_t on_file_detect_err;                /* What to do on dir/file detect error? */
    action_t on_process_dir_ok;                        /* What to do after successfully processing the dir? */
    action_t on_process_dir_err;                       /* What to do if processing the dir failed ? */
    action_file_detection_status_t on_detect_file_ok;  /* What to do when detected a file? */
    action_args_status_t after_process_dir;            /* What to do to a directory after processing all files in it? */

Additional function pointers (only needed when changing the directory tree, such as deleting files and/or subdirectories, see rm-rf-fp.c for usage):

    action_t rewind;                                   /* Optionally, rewinddir(), only needed when applying unlink() and rmdir() */
    action_t reset_flag;                               /* Optionally clear flag for rewind() */
    action_t set_flag;                                 /* Optionally set flag for rewind() */
    action_flag_t get_flag;                            /* Optionally get flag for rewind() */

Note: Regex code influenced by https://rosettacode.org/wiki/Walk_a_directory/Recursively#Library:_POSIX