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
Do not reply on broadcast requests (fixes #153)

According to the Modbus specification
(http://www.modbus.org/docs/Modbus_over_serial_line_V1_02.pdf, section 2.1)
a Modbus RTU master can send a broadcast to all of it's slaves. This
broadcasts can only be write requests as otherwise collisions could
occur, eg. on a RS-485 bus.

When receiving such a broadcast, the slave should process the request as
usual, but must not reply anything, neither a normal response nor an
exception reply in case of an error.

Adjust the unit test for this case, too.

Signed-off-by: Michael Heimpold <mhei@heimpold.de>