(bug 16020) Fix race condition in User::addToDatabase()

(bug 16020) Fix race condition in User::addToDatabase()

Fix the DB error which comes from User::addToDatabase() if it is called
when the user already exists. This is the most common DB error we log at
WMF in normal operation, perhaps because of double clicks on the "create
account" button, or perhaps due to CentralAuth autocreation when
multiple pages on another wiki are opened in the browser simultaneously,
as the bug reporter suggests.

See the doc comment for the interface rationale. Patched
Special:Userlogin to be aware of the new return value. Most extension
callers will continue to work, I will patch a couple that need it in
subsequent commits.

Change-Id: I1f6ef5e6319bfe692fb82a3fa50dc66c9fde8f15