[Dovecot] imap_capability setting

[Bill Boebel]

In src/master/master-settings.c, there is a config variable
"imap_capability" that is read in from the conf file... but in the rest of
the code it looks like that variable is never used.  I assume this is
there for future use, but just wanted to confirm that the code not using
it was intentional.

Since we are using Dovecot as an IMAP proxy for a Courier backend (for
now), we need to advertise a different capability string than the Dovecot
default.  I have changed the hardcoded value in the ./configure script (@
31015) for now.

Bill