* Add 'const' to arguments of some functions. While we're at it, remove the
duplicate extern declaration of FaultInjector_InjectFaultIfSet from
gpdbdefs.h.

* pstrdup() constants passed to makeString(). I think the lack of copy
was harmless, but makeString() explicitly says that the caller should make
a copy, and all other callers seem to obey that, so better safe than sorry.