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
Introduce offsets in modbus mappings

This allows to place the coils/registers at any virtual
start address, not only at address 0. This can be useful e.g.
when the server has to fulfill a specification in which
registers are expected at predefined locations.

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