* src/tgba/acc.cc, src/tgba/acc.hh: Move the strip() routine from
acc_cond into acc_cond::mark_t, and the strip() routine from
cleanacc.cc into acc_cond::acc_code.  Introduce helper functions
to create inf()/fin()/t()/f() at the acc_code level.
* src/tgbaalgos/cleanacc.cc: Simplify, using the strip() function
from acc_code.
* src/tgbaalgos/mask.cc (mask_acc_sets): Use the strip() function
from acc_code so that it work on non-Buchi acceptance.
* src/tgbatest/maskacc.test: Add a test for the latter change.
* src/tgbaalgos/sccfilter.cc, src/tgbatest/acc.cc: Adjust the
use mark_t::strip().