conf: replaced gcc "const" attribute with "pure"

The functions were not "const" (they examined values pointed
by arguments passed to them, quoting gcc's doc: "Note that
a function that has pointer arguments and examines the data
pointed to must _not_ be declared 'const'.") but rather
"pure" and still not all of them.

Note also, that even some of the functions declared "pure"
are not pure, however, due to reasons stated in source code
the attribute has been kept.

src/conf.h

@@ -102,7 +102,6 @@ struct config_param {
  * A GQuark for GError instances, resulting from malformed
  * configuration.
  */
-G_GNUC_CONST
 static inline GQuark
 config_quark(void)
 {
@@ -122,18 +121,25 @@ void config_read_file(const char *file);
 
 /* don't free the returned value
    set _last_ to NULL to get first entry */
-G_GNUC_CONST
+G_GNUC_PURE
 struct config_param *
 config_get_next_param(const char *name, const struct config_param *last);
 
-G_GNUC_CONST
+G_GNUC_PURE
 static inline struct config_param *
 config_get_param(const char *name)
 {
 	return config_get_next_param(name, NULL);
 }
 
-G_GNUC_CONST
+/* Note on G_GNUC_PURE: Some of the functions declared pure are not
+   really pure in strict sense.  They have side effect such that they
+   validate parameter's value and signal an error if it's invalid.
+   However, if the argument was already validated or we don't care
+   about the argument at all, this may be ignored so in the end, we
+   should be fine with calling those functions pure.  */
+
+G_GNUC_PURE
 const char *
 config_get_string(const char *name, const char *default_value);
 
@@ -142,27 +148,31 @@ config_get_string(const char *name, const char *default_value);
  * absolute path.  If there is a tilde prefix, it is expanded.  Aborts
  * MPD if the path is not a valid absolute path.
  */
-G_GNUC_CONST
+/* We lie here really.  This function is not pure as it has side
+   effects -- it parse the value and creates new string freeing
+   previous one.  However, because this works the very same way each
+   time (ie. from the outside it appears as if function had no side
+   effects) we should be in the clear declaring it pure. */
+G_GNUC_PURE
 const char *
 config_get_path(const char *name);
 
-G_GNUC_CONST
+G_GNUC_PURE
 unsigned
 config_get_positive(const char *name, unsigned default_value);
 
-G_GNUC_CONST
+G_GNUC_PURE
 struct block_param *
 config_get_block_param(const struct config_param *param, const char *name);
 
-G_GNUC_CONST
+G_GNUC_PURE
 bool config_get_bool(const char *name, bool default_value);
 
-G_GNUC_CONST
+G_GNUC_PURE
 const char *
 config_get_block_string(const struct config_param *param, const char *name,
 			const char *default_value);
 
-G_GNUC_CONST
 static inline char *
 config_dup_block_string(const struct config_param *param, const char *name,
 			const char *default_value)
@@ -170,17 +180,16 @@ config_dup_block_string(const struct config_param *param, const char *name,
 	return g_strdup(config_get_block_string(param, name, default_value));
 }
 
-G_GNUC_CONST
+G_GNUC_PURE
 unsigned
 config_get_block_unsigned(const struct config_param *param, const char *name,
 			  unsigned default_value);
 
-G_GNUC_CONST
+G_GNUC_PURE
 bool
 config_get_block_bool(const struct config_param *param, const char *name,
 		      bool default_value);
 
-G_GNUC_CONST
 struct config_param *
 config_new_param(const char *value, int line);