Fix remote buffer overflow vulnerability on write requests

Protects against crafted write requests with a large quantity but a small
byte count. If address + quantity was in the mapping space of the server
and quantity greater than the response size, it was possible to crash
the server.

The sleep/flush sequence improves the handling of following requests.
Switch test programs to a BSD license

It concerns only the files in 'tests' directory.
Fix spelling in bandwidth-client.c
Removed function prototype without implementation

Function modbus_receive_from has no implementation in the library and is not used anywhere.
Remove old documentation of modbus_receive_from
Fix crash modbus_new_rtu when baud is 0
Unit test for baud rate check and error message
Move check of device earlier to avoid a free call
Fix remaining GPLv3 header (closes #221)
Update license for the tests in Debian packaging (#221)
unit-test-client: fix compilation on compilers not supporting c99 mode

This fixes the message
"error: 'for' loop initial declarations are only allowed in C99 mode",
spotted during cross-compiling of libmodbus.

The compiler (gcc) actually does support -std=c99 but it gets not
enabled by default. So a solution would be to enforce c99 mode
via CFLAGS, but this will knock out all compilers which do not
support this mode.
Moving the declaration out of the loop initialisation, seems to be
the simpler solution.

Signed-off-by: Michael Heimpold <mhei@heimpold.de>
Prefix hexadecimal values with 0x to avoid confusion
Don't flush on illegal address errors in single write functions

This change has introduced by 83c34102.
Fix unit tests on some systems.
Fix wrong usleep value to wait 5 ms between bytes in test server
Fix too low response timeouts in unit tests

The response timeout of the client must be higher than the server
to handle sleep/flush operations done on server side.
Updated NEWS file
Rewrite documentation macro and Makefile

- only check if documentation tools are installed in acinclude.m4
- don't provide built documentation in dist tarball
- simpler Makefile
- display warning message and check result
Minor adjustments to README.md about documentation
doc: generate manual pages with a2x
doc: slight change in asciidoc config file to print vX.Y