v3.1.3

Many fixes and improvements on data mapping, float handling and RTS.

The structure modbus_mapping_t has been extended to include the new start
addresses (ABI break).

New function: modbus_mapping_new_start_address

• New bswap macros for Max OSX by Jakob Bysewski.
• Fix "wildcard address" in TCP IPv6 by Shoichi Sakane.
• Introduce offsets in modbus mappings with modbus_mapping_new_start_address.
  Thanks to Michael Heimpold and Stéphane Raimbault.
• Fix address range in random-test-client.
  Thanks to Martin Galvan.
• Add an option to disable tests compilation by Yegor Yefremov.
• Define MSG_DONTWAIT to MSG_NONBLOCK on AIX (#294).
  Thanks to Fabrice Cantos.
• Fix building when byteswap.h is not defined by Tomasz Mon.
• Add some more macros for data manipulation and documentation.
• Remove duplicate install of modbus.h (closes #290).
  Thanks to Daniel Sutcliffe.
• Move MIGRATION and README.md to dist_doc_DATA target.
• Change order of few functions in modbus RTU code.
• Add entries for modbus_rtu_[get|set]_delay in documentation index.
• Implemented runtime configurable RTS delay by Jimmy Bergström.
• Add an entry in libmodbus index page for modbus_rtu_set_custom_rts.
• Add support for user defined RTS toggle function by Jimmy
  Bergström.
• Added ILLEGAL_DATA_ADDRESS tests for modbus_write_register[|s].
  Thanks to Andrey Skvortsov.
• Update documentation of modbus_rtu_set_rts
• Fix rts signal switch time by Hiromasa Ihara.
• Improve new_rtu and set_slave documentation (related to #276).
• Fix late check of ctx in modbus_reply[|_exception] (closes #269).
• Wait the server for 1 second before running tests (help Travis).
• A libmodbus context isn't thread safe and won't be (closes #246).
• Fix buffer overflow in modbus_mask_write_register (#265).
• Minor adjustments to README about AsciiDoc.
• Export MODBUS_MAX_ADU_LENGTH and documentation (ref #241).
• Explicit check against Modbus broadcast address.
• Do not reply on broadcast requests (fixes #153). Thanks to Michael.
• Add Travis support.
• Run unit tests with standard: make check (closes #205, closes #238).
  This patch has been developed by Andrey Skvortsov, Michael Heimpold
  and Stéphane Raimbault.
• modbus_send_raw_request: limit request length (fixes #207).
  Thanks to Hanno Neuer for spotting this security flaw.
• Add new contributors to AUTHORS
• Introduce SPDX license identifiers. Thanks to Michael Heimpold.

All changes w/o attribution by @stephane