Add camel_imapx_normalize_mailbox().

Converts the special INBOX mailbox to all caps, if it appears at the
beginning of 'mailbox_name'.  The 'mailbox_name' string is modified in
place.  The 'separator' character helps reliably identify descendants
of INBOX.
Bug 707116 - IMAPX: Normalize descendants of INBOX

camel_imapx_parse_mailbox() was correctly normalizing INBOX itself,
but leaving descendants of INBOX untouched.

If the IMAP server names INBOX as, let's say, "Inbox" and INBOX has
children, then we would end up parsing LIST responses like so:

    LIST "/" "Inbox"        -> "INBOX"
    LIST "/" "Inbox/Child"  -> "Inbox/Child"

This screwed up parent/child relationships in the CamelFolderInfo,
causing "INBOX" and "Inbox" to be represented as separator folders.