disable the tests that fail because of mdi queueing

After the upcoming 2.5.2 release we'll fix the mdi queueing bug and
re-enable these tests.
pncconf -fix error when inverting PWM

sserial analogs caused an error. so we check for it now and dont
 try to invert them.
update comments for Interp::convert_tool_select (M6)

These comments had rotted - the code has evolved but the comments
have not.  This brings the comments back in sync with reality.
io: fix a misleading comment
rename the argument to SELECT_POCKET() to be more clear
add sim tests for the "toolno != pocket number" bug

These tests run G10 L10 and verify the behavior of linuxcnc using numbered
parameters for introspection.

There's currently a bug that results in incorrect TLO when using
nonrandom-tc and the tool number != pocket number.  (random-tc does not
have this bug.)  The fix is in the next few commits after this one.
fix a bug in convert_setup_tool(), aka G10

Tools have both a (constant) tool number and a (possibly variable)
pocket number.  This commit fixes a bug where these two were confused
with each other.

The result of the bug is that running G10 on a tool whose tool-number
differs form its pocket-number, while that tool is in the spindle (as
is done during Axis tool-table touch-off), would fail to update the
active TLO.
remove silly code in Interp::convert_setup_tool()

We just looked up the pocket number based on the passed-in tool number,
so it is a no-op to set the pocket's tool number equal to the passed-in
tool-number.
Interp::convert_setup_tool(): avoid a spurious tool copy

When updating the tool table using G10, on a random tool changer, if the
tool being modified is currently loaded, then a spurious tool info copy
would be performed.

This commit checks if we're on a random tc machine, and if so skips the
tool copy.  (Also adds a comment describing the situation.)
fix a bug in emccanon's GET_EXTERNAL_TOOL_SLOT()

Before this commit GET_EXTERNAL_TOOL_SLOT() used to return the tool
number of the currently loaded tool, which is wrong.  It should return
the *pocket* of the currently loaded tool.

This commit searches the tool table for the currently loaded tool and
returns its pocket number.
comment emccanon's GET_EXTERNAL_SELECTED_TOOL_SLOT()
rename a variable in Interp::convert_tool_length_offset()

Rename "index" to "pocket_number" for clarity.
better comments in Interp::convert_tool_length_offset()
rename a variable in Interp::convert_cutter_compensation_on() for clarity

Instead of "index" call it "pocket_number", which is what it is.  No code
changes beyond the variable rename.
docs: add Code Notes for toolchanger
skip these tests for now

They show that the preceeding bugfix works, but they fail intermittently
due to the mdi queueing bug.  Will turn these tests back on after the
mdi queue fix goes in, after 2.5.2 is released.