patch 8.1.1766: code for writing session file is spread out

Problem:    Code for writing session file is spread out.
Solution:   Put it in one file. (Yegappan Lakshmanan, closes #4728)
diff --git a/src/proto/session.pro b/src/proto/session.pro
new file mode 100644
index 0000000..3c5ee2c
--- /dev/null
+++ b/src/proto/session.pro
@@ -0,0 +1,8 @@
+/* session.c */
+void ex_loadview(exarg_T *eap);
+int write_session_file(char_u *filename);
+void ex_mkrc(exarg_T *eap);
+var_flavour_T var_flavour(char_u *varname);
+int put_eol(FILE *fd);
+int put_line(FILE *fd, char *s);
+/* vim: set ft=c : */