Syntax: USER ABORT [ string ]
or      USER INTEGER string

   USER ABORT generates a syntax error, using the string as the offending
command if it's provided. If the macro abort_handler is defined it'll be
passed the string instead of generating an error --- be sure to define it
with 111 arguments!

   USER integer calls the function "userfn", passing the address of
INTEGER and the rest of the line (as the address of a NUL-terminated
string) as arguments. This may be used to provide user functions
without changing the parser.