Corrected display (on Courier at least) of mailboxes with INBOX

Corrected display (on Courier at least) of mailboxes with INBOX
prefix. INBOX was being added to the tree twice (was already the
root), and then because it was root, the link (with proper nesting)
wasn't being displayed.

Also corrected setting 'noselect' for INBOX (now tree root) -
was being errantly set as 'flag' which was causing errors about
not havign noselect set...

Also removed the places where it was checking whether or not
noselect was set for a mailbox - given the structure, noselect should
always be set - and we should get an error if it isn't (for boxlist
formed from LSUB).

git-svn-id: https://svn.code.sf.net/p/squirrelmail/code/trunk/squirrelmail@5042 7612ce4b-ef26-0410-bec9-ea0150e637f0