Update: numerous changes, most notably path and vfork to fork changes

Update: numerous changes, most notably path and vfork to fork changes

Replace vfork() calls with fork().
- I have determined that vfork() is not safe.
- When calling clearenv() inside a vfork() child process, the parent process' environment ends up getting cleared as well!
- The child can alter the parents memory according to manpages, so stop using vfork() entirely.

Add path processing functions and related defines.

Enable path processing to allow for execvpe()-like behavior can be implemented.
- The execvpe() function is not used because it is not POSIX.
- Manually process the PATH environment to determine what to execute, even when PATH enviornment gets cleared.

Some consistency improvements in defines, such as f_console_max_size to f_console_length_size.

Fix mistake in memcpy usage, dynamic strings use char * for their string so passing & is incorrect.

The f_file_exists() is using access().
- This is misleading and incorect.
- Use stat() to determine if file exists because it doesn't require access to the file (aside from directory access) to check existence.
- Add a new function f_file_access() as the old implementation of f_file_exists().

The fll_execute programs now support additional functions for clearing environment variables.
- This effectively sandboxes the environment variables before calling the program.