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/os/shell.h

@@ -17,11 +17,11 @@ typedef enum {
   kShellOptHideMess = 128,  ///< previously a global variable from os_unix.c
 } ShellOpts;
 
-char ** shell_build_argv(char_u *cmd, char_u *extra_shell_arg);
 
-void shell_free_argv(char **argv);
 
-int os_call_shell(char_u *cmd, ShellOpts opts, char_u *extra_shell_arg);
 
+#ifdef INCLUDE_GENERATED_DECLARATIONS
+# include "os/shell.h.generated.h"
+#endif
 #endif  // NVIM_OS_SHELL_H