Increased the file count to 16 bits when written to the object file.

Increased the file count to 16 bits when written to the object file.
Moved the input file table to a separate module and added an AddFile()
function to add files to this table.
Removed the 8 bit limit for the file number in several places (the file
number is still 8 bits in the file position structure).
Added a pseudo instruction .dbg that will be used to add debug info from
high level code to the assembler source.
Added a subkey "file" to the .dbg command that allows to add a file to the
file table that is later written to the object file.

git-svn-id: svn://svn.cc65.org/cc65/trunk@258 b7a2c559-68d2-44c3-8de9-860c34a00d81