Fix various -Wpointer-compare warnings

[jrtc27]

Summary

This warning (comparing a pointer against a zero character literal
rather than NULL) has existed since GCC 7.1.0, and was recently added to
Clang trunk. Almost all of these are harmless, except for fwcontrol's
str2node, which needs to both guard against dereferencing a NULL
pointer (though in practice it appears none of the callers will ever
pass one in), as well as ensure it doesn't parse the empty string as
node 0 due to strtol's awkward interface.

[brooks]

I've merged the tcsh changes in via an update to tcsh 6.21.00 in rS353325.