liberty: dont_use value is ignored

[bfg86]

Hi,
I reported this in the Yosys repository (3065). The testcase requires Yosys to reproduce, let me know if I should make a testcase for ABC only.

Steps to reproduce the issue

unzip testcase.zip
yosys -s syn.ys

Expected behavior

I added dont_use : "false"; to the NAND gate in cmos_cells.lib.
Expected ABC to not report any dont_use cells, and netlist to contain 1 NAND + 1 NOT.

Actual behavior

In ABC log it says:
ABC: Library "demo" from "/home/test/tmp/cmos_cells.lib" has 3 cells (2 skipped: 2 seq; 0 tri-state; 0 no func; 1 dont_use). Time = 0.00 sec
and the module is implemented with 1 NOR cell and 2 NOT cells.