cpp: enable as many warnings as practical

*** 247,257 ****
  char            *skipbl(char *);
  static  char            *unfill(char *);
  static  char            *doincl(char *);
  static  int             equfrm(char *, char *, char *);
  static  char            *dodef(char *);
! static  char            *control(char *);
  static  struct symtab   *stsym(char *);
  static  struct symtab   *ppsym(char *);
  void            pperror(char *fmt, ...);
  void            yyerror(char *fmt, ...);
  static  void            ppwarn(char *fmt, ...);
--- 247,257 ----
  char            *skipbl(char *);
  static  char            *unfill(char *);
  static  char            *doincl(char *);
  static  int             equfrm(char *, char *, char *);
  static  char            *dodef(char *);
! void                    control(char *);
  static  struct symtab   *stsym(char *);
  static  struct symtab   *ppsym(char *);
  void            pperror(char *fmt, ...);
  void            yyerror(char *fmt, ...);
  static  void            ppwarn(char *fmt, ...);
*** 906,916 ****
  }
  
  #define fasscan() ptrtab=fastab+COFF
  #define sloscan() ptrtab=slotab+COFF
  
! static char *
  control(p) register char *p; {/* find and handle preprocessor control lines */
          register struct symtab *np;
  for (;;) {
          fasscan(); p=cotoken(p); if (*inp=='\n') ++inp; dump();
          sloscan(); p=skipbl(p);
--- 906,916 ----
  }
  
  #define fasscan() ptrtab=fastab+COFF
  #define sloscan() ptrtab=slotab+COFF
  
! void
  control(p) register char *p; {/* find and handle preprocessor control lines */
          register struct symtab *np;
  for (;;) {
          fasscan(); p=cotoken(p); if (*inp=='\n') ++inp; dump();
          sloscan(); p=skipbl(p);