modbus: update to 3.1.12

modbus: update to 3.1.12

Update modbus to version 3.1.12

https://github.com/stephane/libmodbus/releases/tag/v3.1.12
 Fix FD_SET overflow when socket fd >= FD_SETSIZE.
 Check dest pointer not null and nb in read functions.
 NULL check for src and nb < 1 validation in write functions.
 modbus_reply: don't compute address for FC 0x07/0x11.
 Use O_NONBLOCK instead of deprecated O_NDELAY.
 Explicit cast for Coverity CID 416366.
 Document required buffer size of modbus_receive.
 Document macros for error codes corresponding to Modbus exceptions.
 Fix example of modbus_rtu_set_serial_mode.
 Test filesystem provides symlink in autogen.sh.
 Sync API signatures with the documentation.
 Many documentation fixes and typo corrections.
 Add coverage target and helper script.

Signed-off-by: Hannu Nyman <redacted>