Constify

fixes:

config.c: In function 'conf_ParseLine':
config.c:206:13: warning: assignment discards 'const' qualifier from pointer target type [enabled by default]
         *op = "+=";
             ^
config.c:210:13: warning: assignment discards 'const' qualifier from pointer target type [enabled by default]
         *op = "=";
             ^

src/config.c

@@ -190,7 +190,8 @@ conf_Add(const char *lval, const char *op, const char *rval)
 }
 
 static int
-conf_ParseLine(char *ptr, char **lval, char **op, char **rval)
+conf_ParseLine(char *ptr,
+    const char **lval, const char **op, const char **rval)
 {
     char *endlval;
 
@@ -311,7 +312,7 @@ CONF_ReadFile(const char *file) {
         VSB_cat(orig, ptr);
         VSB_finish(orig);
 
-        char *lval, *op, *rval;
+        const char *lval, *op, *rval;
         if (conf_ParseLine(ptr, &lval, &op, &rval) != 0) {
             fprintf(stderr, "Cannot parse %s line %d: '%s'\n", file, linenum,
                     VSB_data(orig));