Partially revert r367756 (chpass(1) synopsis changes)

Let's have two entries in the synopsis:

• chpass now lists options which can be used for non-NIS-specific functionalities.
• ypchpass additionally lists the NIS-specific flags.

Technically, it is an artificial distinction, as chpass and ypchpass behave
identically. Nevertheless, it might help navigating the synopsis section.

Reviewed by: imp
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D27251