Constify command options

Reviewed-by: Kurt Roeckx <kurt@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/1694)

apps/errstr.c

diff --git a/apps/errstr.c b/apps/errstr.c
index 5fda799..6bbbe2f 100644
--- a/apps/errstr.c
+++ b/apps/errstr.c
@@ -20,7 +20,7 @@
     OPT_ERR = -1, OPT_EOF = 0, OPT_HELP
 } OPTION_CHOICE;
 
-OPTIONS errstr_options[] = {
+const OPTIONS errstr_options[] = {
     {OPT_HELP_STR, 1, '-', "Usage: %s [options] errnum...\n"},
     {OPT_HELP_STR, 1, '-', "  errnum  Error number\n"},
     {"help", OPT_HELP, '-', "Display this summary"},