Introduce a constant, ELF_NOTE_ROUNDSIZE, which evidently declare our
intention to use 4-byte padding for elf notes.

MFC after:	3 weeks
KVM method support for procstat_getgroups, procstat_getumask,
procstat_getrlimit, and procstat_getosrel.

MFC after:	3 weeks
procstat_getpathname: for kvm method, instead of returning the error
that the method is not supported, return an empty string.

This looks more handy for callers like procstat(1), which will not
abort after the failed call and still output some useful information.

MFC after:	3 weeks