bug#5387: MS950 alias for CP950 charset

[jidanni]

I signed up on the Legislature of Taiwan's website, and the confirmation
mail had
From: sysop@ly.gov.tw
Subject: =?BIG5?B?pd+qa7B8pf6yebjqsFS69C2/76XBqkGwyLHSsMq9VLt7qOc=?=
Mime-Version: 1.0
Content-Type: text/html; charset=MS950
Content-Transfer-Encoding: quoted-printable

And it turns out MS950 is an alias for CP950, so perhaps emacs should
incorporate this alias, even though this is the first time I've seen it.
Perhaps make all MSxxx be aliases for CPxxx.