Cleanup and static variable reduction

Replaced some unecessary static variables with local variables.
Refactored loop() code to use a 1 byte index instead of a pointer.
Simplified handling of checksums with string arguments. Removed some
redundant string parsing code for M28 & M30. Fixed bug in handling of
Gcode commands with trailing comments (would cause next line to be
treated as comment). Moved some advanced config out of the "Buffers"
section and into "Additional Features" section. Fixed earlier EOL
problems in 3 files.