Add the vpiFile and vpiLineNo properties to system functions.
Most other objects have stubs that return "N/A"/0. Interactive
functions (called from the debugger) use <interactive> for the
file name.
Gets rid of a few warning: deprecated conversion from string
constant to 'char*', follows IEEE 1364-2001C 27.10 in more cases,
and fixes at least one real bug (look at the previous use of
strdup/strcat in real_var_get_str() and signal_get_str()).