Fix incorrect test of select() return code

Thanks to Thomas Stalder and Torello Querci for the report.
Fix typo in typedef

Signed-off-by: Michael Heimpold <mhei@heimpold.de>
Add basic information about installation
Fix OMG bug in modbus_mapping_free not freeing memory

Thanks to Stefan Finzel for the bug report
Fix a missing free in random-test-client

Thanks again to Stefan Finzel.
It would be nice to run Valgrind more often...
I was certainly under the influence of drugs or hurry
Sort headers in AC_CHECK_HEADERS
Display node and service in PI and port in IPv4 at connection
poll.h is useless to compile TCP backend
serial_mode isn't a bitmask
Avoid ioctl call on non-RS485 ports

Original patch provided by Michael Haberler <git@mah.priv.at>.
Add -lsocket to compile on QNX
Fix another Visual C++ 2008/2010 deficiency (closes #53)

Thanks to oldfaber.
Fix TCP PI init under Windows

Thanks to oldfaber
Return value of _modbus_tcp_pi_connect() on failure (closes #61)
Fix compilation warnings with Microsoft compiler (closes #64)
Init os_qnx in configure.ac
Fix random-test-* with Microsoft compiler (closes #65)
Remove useless test on error_recovery argument

Thanks to Carlos Tangerino
Bump version number to 3.1.0 and library version