Miscellaneous minor fixes/improvements.

Miscellaneous minor fixes/improvements.
Most notable is that the rundiff trace-output lossage is fixed!

base/pollevent.cc:
    Use SA_RESTART to keep blocked write() calls from failing
    with EINTR when signals happen.
base/trace.hh:
    Make TRACING_ON depend on NDEBUG not being set (instead of
    DEBUG being set) so m5.opt/m5.fast do what we want.
util/rundiff:
    Print header lines showing file/cmd names.

--HG--
extra : convert_revision : 4966814ad65aedb6d62a5a62a6ad3e3f326d61b8