Add member to Process to retrieve the child's exit code after, e.g., inserting
the eoi manipulator of after calling eoi().

Add member to OneKey(?) to switch off/on cin-input TTY echoing.

Add a member to Arg to call help from the Arg object

Provide examples in the following man-pages:

ifdstreambuf.yo
milter.yo
ofdstreambuf.yo
