Added include_inbox_namespace to specify inbox namespace

For some reason, dovecot insists on having a namespace with "inbox=yes".
I implemented this as simply as possible :-)

My resulting 10-mail.conf
# cat /etc/dovecot/conf.d/10-mail.conf | grep -v ^".*#" | grep -v ^$ 
mail_location = mbox:~/mail/mailboxes:DIRNAME=mbox:INDEX=~/mail/index:CONTROL=~/mail/control
namespace inbox {
  inbox = yes
}
mbox_write_locks = fcntl

If you want it implemented differently, do tell, and I will try to accommodate you.