commands: change API of "make_seed" and "create" commands

instead of "segwit" boolean, take a "seed_type" optional arg
default seed_type to "segwit"
previously these commands created legacy seeds by defalt

electrum/tests/test_mnemonic.py

@@ -120,7 +120,7 @@ class Test_NewMnemonic(SequentialTestCase):
         iters = 10
         m = mnemonic.Mnemonic(lang='en')
         for _ in range(iters):
-            seed = m.make_seed()
+            seed = m.make_seed("standard")
             i = m.mnemonic_decode(seed)
             self.assertEqual(m.mnemonic_encode(i), seed)