Add support for active-high limit switches

A friend of mine wanted to use some optical interrupter modules that signaled high when their beam was broken. A minor software change was required to accomplish this, and this patch surfaces that change as a setting. Configuring the new variable $23 as "true" will cause grbl to enable active-high limit switch support (internal pullups disabled, homing cycle inverts invert_pin), while "false" (the default) will leave grbl with its original behavior for supporting active-low switches.