v3.1.0

OK last year, I've silently released libmodbus v3.1.0 and it's never too late to
announce it!

Major changes to handle many slaves in RTU mode, non blocking connections and
RTS flow control.

• Fixes for Microsoft Visual C++ compiler
• Fixes for Windows. Thanks to oldfaber
• Return value of _modbus_tcp_pi_connect() on failure (closes )
• Avoid ioctl call on non-RS485 ports. Thanks to Michael Haberler
• Display node and service in PI and port in IPv4 at connection
• Return -1 on getaddrinfo error and print error in debug mode
• More robust way to establish the connection in non blocking mode
• TCP - Socket in non blocking mode by default. Thanks to Thomas Stalder
• Apply CLOEXEC flag for TCP protocol independent too (IPv6)
• New RTU receive() to ignore confirmation from other slaves (closes #18)
• Move RTU filtering in CRC check to avoid useless call to modbus_reply
• Unique transaction identifier by TCP connection
• Use accept4 with SOCK_CLOEXEC when available (Linux)
• Open fd and socket with the CLOEXEC flag when available
• Exception response on report slave ID wasn't detected (closes #27)
• Provides a way to disable the byte timeout (Alex Stapleton)
• Added slave ID check for response messages (Alex Stapleton)
• RTS flow control with modbus_rtu_set_rts and modbus_rtu_get_rts functions by Torello Querci and Stéphane Raimbault.