This fix addresses an issue I saw with the ASAP7 liberty files and
ABC. ASAP7 lists internal pins in its liberty file which ABC's liberty
parser doesn't account for. This causes an assert to be triggered. This
fix simply adds interal pins to the ignore list.

Example found in the 2007.03 Liberty Reference Manual that was also found
in the wild:

    input_voltage(CMOS) {
        vil : 0.3 * VDD ;
        vih : 0.7 * VDD ;
        vimin : -0.5 ;
        vimax : VDD + 0.5 ;
    }

Current implementation just parses the expression but no interpretation is done.

Restoring constraint manager to read old constraint file by default (use 'read_constr -n' to read new format).