Changes:
* IMAP mailboxes can now be cached locally. See the string variable
'imap-cache' in the manual page. The cache can also be used for
disconnected operation mode, see 'disconnected' and also the '-D'
command line option.
* When nail is notified by the server that messages have been expunged,
the user is informed about this condition even if new messages have
also arrived.
* When the group of variables for an 'account' command contains a variable
prefixed by 'no...' and the related variable is not set, it is not an
error to unset it.
* The 'headers' command has been made faster when operating on an IMAP
mailbox by bundling IMAP requests.
* When a 'folder' command was executed to change from an uncompressed
file to a compressed one, the latter was incorrectly detected as
being read-only.
* Unless one of the 'autoinc' or 'newmail' variables is set, nail does
not actively check for new messages in the current IMAP mailbox before
each prompt. New messages may nevertheless be detected after any
other IMAP command has been issued, so the effect is mostly that
not every nail command causes an IMAP command to be sent. Also if
'autoinc' or 'newmail' are set to the value "noimap", they are
ignored for IMAP mailboxes but still effective for local folders.
* The 'inc' and 'newmail' commands now use the same method to determine
the location of the current message as the 'folder' command does. That
means if there has been new mail before that is still unread when newer
mail arrives, the older messages are displayed first.
* Nail does not generate 'Sender:' fields anymore.
* If OpenSSL is not available, STARTTLS support is not built, and nail
compiles again without OpenSSL.
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