patch 9.0.1064: code for making 'shortmess' temporarily empty is repeated

Problem:    Code for making 'shortmess' temporarily empty is repeated.
Solution:   Add functions for making 'shortmess' empty and restoring it.
            (Christian Brabandt, closes #11709)
diff --git a/src/proto/optionstr.pro b/src/proto/optionstr.pro
index 1fa6c56..e75e3dc 100644
--- a/src/proto/optionstr.pro
+++ b/src/proto/optionstr.pro
@@ -11,4 +11,6 @@
 char *set_string_option(int opt_idx, char_u *value, int opt_flags);
 char *did_set_string_option(int opt_idx, char_u **varp, char_u *oldval, char *errbuf, int opt_flags, int *value_checked);
 int check_ff_value(char_u *p);
+void save_clear_shm_value(void);
+void restore_shm_value(void);
 /* vim: set ft=c : */