This routine will find an entry in a nested value option or configurable.
If "valueName" is NULL, then the first entry is returned. Otherwise,
the first entry with a name that exactly matches the argument will be
returned.
pOptValue
a hierarchcal value
valueName
name of value to get
RETURN VALUE
a compound value structure
ERRORS
The returned result is NULL and errno is set:
EINVAL - the pOptValue does not point to a valid
hierarchical option value.
ENOENT - no entry matched the given name.
@end itemize