value < 0. errno is only updated in this case. Change all (most?)
callers of syscalls to follow this better, and let's see if this strictness
helps us in the future.
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.
jsg