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.c

@@ -31,20 +31,13 @@ typedef struct {
   garray_T ga;
 } ProcessData;
 
-static int tokenize(char_u *str, char **argv);
 
-static int word_length(char_u *command);
+#ifdef INCLUDE_GENERATED_DECLARATIONS
+# include "os/shell.c.generated.h"
+#endif
 
-static void write_selection(uv_write_t *req);
 
-static int proc_cleanup_exit(ProcessData *data,
-                             uv_process_options_t *opts,
-                             int shellopts);
 // Callbacks for libuv
-static void alloc_cb(uv_handle_t *handle, size_t suggested, uv_buf_t *buf);
-static void read_cb(uv_stream_t *stream, ssize_t cnt, const uv_buf_t *buf);
-static void write_cb(uv_write_t *req, int status);
-static void exit_cb(uv_process_t *proc, int64_t status, int term_signal);
 
 /// Builds the argument vector for running the shell configured in `sh`
 /// ('shell' option), optionally with a command that will be passed with `shcf`