Make string_to_hex/hex_to_string public

Give the API new names, document it.

Reviewed-by: Richard Levitte <levitte@openssl.org>
diff --git a/apps/cms.c b/apps/cms.c
index a74ca9d..95f2124 100644
--- a/apps/cms.c
+++ b/apps/cms.c
@@ -453,7 +453,7 @@
             noout = print = 1;
             break;
         case OPT_SECRETKEY:
-            secret_key = string_to_hex(opt_arg(), &ltmp);
+            secret_key = OPENSSL_hexstr2buf(opt_arg(), &ltmp);
             if (secret_key == NULL) {
                 BIO_printf(bio_err, "Invalid key %s\n", opt_arg());
                 goto end;
@@ -461,7 +461,7 @@
             secret_keylen = (size_t)ltmp;
             break;
         case OPT_SECRETKEYID:
-            secret_keyid = string_to_hex(opt_arg(), &ltmp);
+            secret_keyid = OPENSSL_hexstr2buf(opt_arg(), &ltmp);
             if (secret_keyid == NULL) {
                 BIO_printf(bio_err, "Invalid id %s\n", opt_arg());
                 goto opthelp;