docs: add subsections for servos & steppers

The AXIS_n section of the INI docs was a bit confusing.  It started
with generic ini variables applicable to all axis configs, then had a
subsection for homing.  In the homing section it then continued with
ini variables for servos and steppers.  This was confusing.

This commit adds a subsection for Servos and one for Steppers, after
the Homing subsection.  Hopefully this will be clearer.
skip all the tests that use linuxcncrsh

There is some strange bug lurking in linuxcncrsh that causes these tests
to sometimes hang.  I'm disabling them until I figure out what's wrong.
run nc correctly

The -z argument turns on "scan for listeners" mode, which turns off
"pass traffic" mode.  This is true on both netcat-traditional (which is
the default netcat in Ubuntu Precise) and in netcat-openbsd, which is
the other one that's available in Precise.

Ahem.  ;-)