dgst: using digest instead of specific digest commands
the digest list specified in man dgst may be inaccurate, hence using
digest and referring to the list in digest-commands

'sha' as a digest name is no longer supported

dgst,pkeyutl cmds help cleanup
- In dgst, pkeyutl cmds, some options help was missing.
- fixed a minor typo in openssl.pod, that fixes make install.
- digest-commands was showing ‘sha’, which is not a supported digest
anymore.
Signed-off-by: NRich Salz <rsalz@openssl.org>
Reviewed-by: NTim Hudson <tjh@openssl.org>