Introduce nvim namespace: Move files.

Move files from src/ to src/nvim/.
- src/nvim/ becomes the new root dir for nvim executable sources.
- src/libnvim/ is planned to become root dir of the neovim library.

src/nvim/main.h

@@ -0,0 +1,16 @@
+#ifndef NEOVIM_MAIN_H
+#define NEOVIM_MAIN_H
+
+#include "normal.h"
+
+void main_loop(int cmdwin, int noexmode);
+void getout(int exitval);
+int process_env(char_u *env, int is_viminit);
+void mainerr_arg_missing(char_u *str);
+void time_push(void *tv_rel, void *tv_start);
+void time_pop(void *tp);
+void time_msg(char *mesg, void *tv_start);
+char_u *eval_client_expr_to_string(char_u *expr);
+char_u *serverConvert(char_u *client_enc, char_u *data, char_u **tofree);
+
+#endif /* NEOVIM_MAIN_H */