Fix various -Wpointer-compare warnings

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.

Submitted by: James Clarke <jtrc27@jrtc27.com>
Obtained from: CheriBSD
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D21914