Add automatic generation of headers

- The 'stripdecls.py' script replaces declarations in all headers by includes to
  generated headers.
  `ag '#\s*if(?!ndef NEOVIM_).*((?!#\s*endif).*\n)*#ifdef INCLUDE_GENERATED'`
  was used for this.
- Add and integrate gendeclarations.lua into the build system to generate the
  required includes.
- Add -Wno-unused-function
- Made a bunch of old-style definitions ANSI

This adds a requirement: all type and structure definitions must be present
before INCLUDE_GENERATED_DECLARATIONS-protected include.

Warning: mch_expandpath (path.h.generated.h) was moved manually. So far it is
the only exception.

src/nvim/misc2.h

@@ -4,34 +4,7 @@
 #include "nvim/func_attr.h"
 #include "nvim/os/shell.h"
 
-/* misc2.c */
-int virtual_active(void);
-int inc(pos_T *lp);
-int incl(pos_T *lp);
-int dec(pos_T *lp);
-int decl(pos_T *lp);
-int csh_like_shell(void);
-int copy_option_part(char_u **option, char_u *buf, int maxlen,
-                     char *sep_chars);
-int get_fileformat(buf_T *buf);
-int get_fileformat_force(buf_T *buf, exarg_T *eap);
-void set_fileformat(int t, int opt_flags);
-int default_fileformat(void);
-int call_shell(char_u *cmd, ShellOpts opts, char_u *extra_shell_arg);
-int get_real_state(void);
-int vim_chdirfile(char_u *fname);
-int illegal_slash(char *name);
-int vim_chdir(char_u *new_dir);
-int emsg3(char_u *s, char_u *a1, char_u *a2);
-int emsgn(char_u *s, int64_t n);
-int emsgu(char_u *s, uint64_t n);
-int get2c(FILE *fd);
-int get3c(FILE *fd);
-int get4c(FILE *fd);
-time_t get8ctime(FILE *fd);
-char_u *read_string(FILE *fd, int cnt);
-int put_bytes(FILE *fd, long_u nr, int len);
-void put_time(FILE *fd, time_t the_time);
-int has_non_ascii(char_u *s);
-
-#endif /* NVIM_MISC2_H */
+#ifdef INCLUDE_GENERATED_DECLARATIONS
+# include "misc2.h.generated.h"
+#endif
+#endif  // NVIM_MISC2_H