Misc cleanups and performance improvements.

- There's no need to check rval for prop_dictionary_get_*, we are sure the
  objects are there at prop_dictionary_set_* time.
- Avoid two chdir(2) calls per INSTALL/REMOVE run.
- Avoid using access(2) to check for existence of INSTALL/REMOVE scripts,
  just try to run the executable directly and check for ENOENT.

lib/package_state.c

@@ -83,8 +83,8 @@ get_state(prop_dictionary_t dict)
 
 	assert(dict != NULL);
 
-	if (!prop_dictionary_get_cstring_nocopy(dict, "state", &state_str))
-		return 0;
+	prop_dictionary_get_cstring_nocopy(dict, "state", &state_str);
+	assert(state_str != NULL);
 
 	if (strcmp(state_str, "unpacked") == 0)
 		state = XBPS_PKG_STATE_UNPACKED;