isync: Update to 1.4.0

* Import upstream patch to fix runtime errors.

Changelog:
This is a feature release that comes with significant new functionality:

- The IMAP '$Forwarded' / Maildir 'P' (passed) flag is supported now.

- Support for configuring a TLS cipher string was added.

- IMAP mailbox subscriptions are supported now.

- The IMAP user query can be scripted now.

- Added built-in support for the macOS Keychain.

- Messages excluded by MaxSize will now result in placeholders.

Compatibility concerns:

- The 'isync' compatibility wrapper was removed.

- A C11 compiler is required for building now.

- The validity of the config file is checked more stricly now, including:
  - Appearance of options in unexpected places
  - The capitalization of INBOX

- The new TLSv1.3 flag must be added to SSLVersions if the option is
  used, unless disabling that version is desired (which is unlikely).

- Removed support for the obsolete/insecure SSL v3.

- The use of Master/Slave terminology has been deprecated.

Bugfixes:

- All bugfixes up to 1.3.4 are included.
- IMAP protocol errors are handled more robustly now.
- Fixed support for SASL's built-in EXTERNAL mechanism.
- Improved reliability of synchronization when resuming interrupted runs.
- Fixed MaxSize being ignored under certain circumstances when only one of
  New and ReNew was requested.
- Fixed a network inefficiency occurring with server-side mailboxes that
  receive new messages only via mbsync.

(ryoon)