Add support for using - as shorthand for stdin/stdout in tradcpp.

Add support for using - as shorthand for stdin/stdout in tradcpp.

When looking into switching the /usr/bin/cpp wrapper to
tradcpp I came across "| ${CPP} ${CPPFLAGS} -" in usr.bin/which.

gcc documents this behaviour for cpp here:
https://gcc.gnu.org/onlinedocs/cpp/Invocation.html

Versions of the John F. Reiser derived cpp in 32V and CSRG SCCS
accept this usage as well, as does the cpp in PCC.