common: quit using `!` as comment out symbol in config files

It is not used anywhere in default config. Some config like
`tls_ciphers` might contain `!` like this:

    tls_ciphers=FIPS:!aNULL:!eNULL

Fixes #1033.

common/file.c

@@ -138,7 +138,7 @@ file_read_ini_line(struct stream *s, char *text, int text_bytes)
     while (c != 10 && c != 13)
     {
         /* these mean skip the rest of the line */
-        if (c == '#' || c == '!' || c == ';')
+        if (c == '#' || c == ';')
         {
             skip_to_end = 1;
         }