Add $browser_abbreviate_mailboxes.

This allows '=' and '~' mailboxes shortcuts to be turned off in the
browser mailbox list.

The default $sort_browser value of "alpha" uses strcoll(3), which
ignores some punctuation.  When using multiple accounts with
dynamically changing values of $folder, this can lead to an
unintuitive sorting order.  (See issue #22 for an example.)

This problem came about because of commit 8328ce68, which enabled
mutt_pretty_mailbox() calls for IMAP and POP mailboxes too.  The
commit had a valid point about consistency and too-wide mailbox
listings, so I don't really want to revert it.

This option provides an alternative to completely enable/disable the
shortcuts for those who run into an issue.
4 jobs for master in 13 minutes and 50 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #63997741
gnutls

00:12:23

passed #63997740
openssl

00:12:23

 
  Deploy
passed #63997742
pages

00:01:27

passed pages:deploy

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