portage.const: restore two dropped features

The script used to clean this up accidentally dropped two features.

Reported-by: Sebastian Luther <SebastianLuther@gmx.de>
diff --git a/pym/portage/const.py b/pym/portage/const.py
index a19728b..1785bff 100644
--- a/pym/portage/const.py
+++ b/pym/portage/const.py
@@ -150,6 +150,7 @@
 	"keeptemp",
 	"keepwork",
 	"lmirror",
+	"merge-sync",
 	"metadata-transfer",
 	"mirror",
 	"multilib-strict",
@@ -184,6 +185,7 @@
 	"test-fail-continue",
 	"unknown-features-filter",
 	"unknown-features-warn",
+	"unmerge-backup",
 	"unmerge-logs",
 	"unmerge-orphans",
 	"userfetch",