Docs: fix typo option number

Signed-off-by: John Thornton <jthornton@gnipsel.com>
Docs: add info about thread requirements

Signed-off-by: John Thornton <jthornton@gnipsel.com>
docs: fix mistakes in the gs2 manpage

The long arguments need two dashes, not one.
Merge branch 'v2.5_branch'
build-depend on libmodbus-dev (3.0 or newer)

Currently, linuxcnc uses three different modbus implementations:

    gs2_vfd uses a local copy of libmodbus2 (forked into the linuxcnc
    repo in 2008, hacked with bug fixes over the years)

    vfs11_vfd and mb2hal use libmodbus3, but it was until recently not
    available on Hardy or Lucid (see below)

    classicladded uses its own internal modbus implementation

I took the libmodbus3 source package from Precise, applied some
tiny tweaks to get it to build on Hardy and Lucid, and built it for
those platforms.  The resulting binary debs are now available in the
linuxcnc.org debian archive.

This commit changes linuxcnc to build-depend on the libmodbus package.
It uses the upstream libmodbus3 on Precise, and these new local backported
packages on Hardy and Lucid.  It looks to me like all our local bugfixes
have gone into libmodbus3 (the newly available packaged version).
configure: require libmodbus3

By default, configure will now look for libmodbus (installed from a
separate deb, provided by Ubuntu for Precise and by the linuxcnc.org
debian archive for Hardy and Lucid) and error out if it can't be found.

The user can disable this check (and disable compilation of all drivers
that use libmodbus) by configuring with "--without-libmodbus".
switch gs2_vfd to use libmodbus3

Before this commit, gs2_vfd used our old forked/hacked copy of libmodbus2.

This commit changes gs2_vfd to use the new libmodbus3 API instead.
The change to the C code is largely mechanical.

Also some tweaks to the makefile:

    Don't build gs2 if the user configured with --without-libmodbus.

    Use the libmodbus CFLAGS and LIBS that configure determined.
remove the old forked modbus code

Now that gs2_vfd is updated to build against libmodbus3 (supplied by
official debian packages for Precise, and by linuxcnc backported debian
packages for Hardy and Lucid) there are no users of our forked old copy
of modbus, so remove it.
gs2: simplify --verbose/--debug

The code makes no distinction between --debug and --verbose, so
squash those arguments together.  Leave both for now, for backwards
compatibility.

Also fixes a compiler warning about the variable "verbose" being set
but not used.
fix an "uninitialized variable" warning