* "G1 X1 ; test" was not executing "G1 X1" due to never leaving comment mode.
* "M117 Hello \;)" printed "Hello \" to the display due to not replacing serial_char properly.
Tested with the following commands:
* M117 Hello ; test => displays "Hello" on display, ignores "test"
* G1 X1 ; foo => moves 1mm in X, ignores "foo"
* ; test => completely ignored, not even acknowledged
* M117 Hello \;) => displays "Hello ;)" on display
* M117 Hello \\;) => displays "Hello \" on display, ignores ")"
