v2.9.4: Bump version to 2.9.4

This minor release contains many new features with IPv6 support, documentation
(man and HTML), error recovery modes and small API changes. I'm intend to
release the version 3.0 so I need feeback from this release even more if you use
libmodbus on Windows or MacOS X (check nanosleep support for example).

It's easy to test the library, just compile and run the unit tests! Don't forget
to notify your success or failure (by mail, wiki or bug report), see the
Call for Testers page or the mailing list for details.

There are already Debian and Fedora packages but I need help to submit them to the distributions,
so all experience in this field is appreciated.

• IPv6 support Make the TCP implementation "protocol independent" by Florian Forster
  and Stéphane Raimbault.

• Fix compilation on Windows 7 (x64) with MinGW/MSYS and GCC 4.
  Reported by Patsy Kaye.

• Documentation of libmodbus functions with AsciiDoc (man and HTML) by Stéphane Raimbault

• Avoid an iteration in flush function

• New functions to send and receive raw requests
  (modbus_send_raw_request, modbus_receive_confirmation)

• Fix flush function of TCP backend on Windows

• API changes for server/slave:

  • modbus_receive doesn't take socket/fd argument anymore
  • new function modbus_set_socket to set socket/fd

• API changes for timeout functions:

  • modbus_get_timeout_begin -> modbus_get_response_timeout
  • modbus_set_timeout_begin -> modbus_set_response_timeout
  • modbus_get_timeout_end -> modbus_get_byte_timeout
  • modbus_set_timeout_end -> modbus_set_byte_timeout

• Fix longstanding limitation of server to wait forever

• New functions modbus_set/get_serial_mode by Manfred Gruber and Stéphane
  Raimbault for RS485 communications

• Improved recovery mode (see modbus_set_error_recovery
  documentation) for data link and protocol errors.

• Fix compilation issue with Microsoft Visual Studio 2008. Reported by Allan Cornet.