Fixed compiler segfault on label outside program

When accidentally forgetting the parentheses around an empty argument
list when defining a program, the compiler would interpret this as an
attempt at declaring a label, and would then crash when attempting to
get the current code position, since there would be no program and no
coder...!

Added a check, and a new error code A2_NEXPLABEL.
Minor code cleanups and fixes

Fixed some comment formatting that was screwed up after the
ChipSound to Audiality 2 rename.

The struct size dump is now wired to the DUMPSIZES debug define
rather than DEBUG, and the printout now happens in a2_Open().

Added an a2_Now() call in a2_Open() for initialization.

Compiler symbol debug output now wired to SYMBOLDBG.