Migrated man page post(8) to spost(8).

It would be nice to have it improved a bit more.
Spost is located in section 8 (maintenance commands) of the manual,
but it is located in the bindir, rather than the sbindir. This is
on purpose. The sections of the manual are mostly informational (today),
whereas directories are either in $PATH or not, and thus cause a
functional difference. Hence, I don't blindly link sbindir to section 8.