Raise WARNS of newfs to 6:

Raise WARNS of newfs to 6:
- Use static char buffer instead of string constant;
- Move and add some function prototypes;
- Don't reuse a scratch variable, use a real pid_t to hold the
  result of a fork();
- Cast variables as needed (mainly to u_long and u_int32) to avoid
  signed-unsigned comparison warnings;
- Rename some variables to avoid name clashes;
- Mark some parameters as unused;
- Add MOPT_NULL as a struct mntopt constant for terminating a list.

And apply some style(9):
- Move out some loop invariant code, for clarity;
- Remove `register' keywords;
- Make explicit some checks against NULL.

Based-on-patch-by: Peter Schuller
                   <peter_dot_schuller_at_infidyne_dot_com>
Tested-by: typical usage of installer