Added Inkwell lightpen drivers for the C64 and the C128.

They use the mouse driver programming interface.

Added a test program for lightpen drivers.  Now, it knows about only those two drivers; it will need updating when others are added.
Replaced an obsolete spelling with the current one.
Added library reference address to lightpen driver header.
Used a library-reference method to calibrate lightpen drivers.

The mouse reference is a pointer.  If it's NULL, the driver uses a default.  If it's non-NULL, then it points to a function that the driver can call.  That function will adjust the driver's calibration value.  It could ask the user to adjust the pen; or, it could read a file that holds a value from a previous calibration.

Application writers can choose how it's done: a function that's provided by the library, a custom function, or nothing.
Renamed the mouse sample program because it also tests the mouse library.
Merged testcode/lib/pen-test.c into the mousetest.c sample program.

A command-line option can force mousetest to be built with a statically linked
driver.
Included a header that defines NULL.
Merge branch 'master' into lightpen
Look for NULL pointer more efficiently.
fix '_sysrmdir' for SpartaDOS
Moved common symbol names out of sources, and into a global include file.
Normalized some hexadecimal addresses.
Merge pull request #21 from groessler/something_to_pull

fix '_sysrmdir' for SpartaDOS
implement _sysrename for Atari
Don't import tmp1, slipped in by mistake in last commit
fix cleanup of stack
Merge pull request #22 from groessler/something_to_pull

Something to pull
add "tinyshell" test program for file operations
Merge pull request #23 from groessler/something_to_pull

add "tinyshell" test program for file operations
Moved the lightpen names from <mouse.h> to a new <pen.h>.

Renamed the adjuster pointer.