sysctl: replaces some slashes with dots

It turns out that plain sysctl understands a.b/c syntax to write to
/proc/sys/a/b.c. Support this for compatibility.

https://bugs.freedesktop.org/show_bug.cgi?id=77466

src/sysctl/sysctl.c

@@ -48,12 +48,26 @@ static const char conf_file_dirs[] =
 #endif
         ;
 
-static char *normalize_sysctl(char *s) {
+static char* normalize_sysctl(char *s) {
         char *n;
 
-        for (n = s; *n; n++)
+        n = strpbrk(s, "/.");
+        /* If the first separator is a slash, the path is
+         * assumed to be normalized and slashes remain slashes
+         * and dots remains dots. */
+        if (!n || *n == '/')
+                return s;
+
+        /* Otherwise, dots become slashes and slashes become
+         * dots. Fun. */
+        while (n) {
                 if (*n == '.')
                         *n = '/';
+                else
+                        *n = '.';
+
+                n = strpbrk(n + 1, "/.");
+        }
 
         return s;
 }