Fix i386 build of chpass after r363992

My change to allow bootstrapping pwd_mkdb (r363992) resulted in i386 build
failures because the bootstrap header was being included in non-bootstrap chpass.
Dropping the no longer required pwd_mkdb include path from chpass fixes
the build, but to be certain that the failure doesn't get re-introduced,
I've also moved the bootstrap pwd.h into a subdirectory so that adding
-I${SRCTOP}/usr.sbin/pwd_mkdb doesn't pull it in.

Reported by: mjg