Unify kvno option documentation

Unify kvno option documentation

Add missing kvno options to the kvno.rst synopsis and option
descriptions, and to the kvno usage message.  Remove mention of '-h'
(help text), from kvno.rst as it is an implicit option.  Note that the
three new caching options were added in release 1.19.

Indicate the two exclusions (-u/-S and --u2u with the S4U2Self options)
and dependency (-P on S4U2Self) where they are missing.

Switch xusage() to print only a single localized string, rather than
running each line of output through localization separately.

Leave kvno -C undocumented for now, as the semantics of
KRB5_GC_CANONICALIZE are minimally useful and likely to change.

[ghudson@mit.edu: edited documentation and commit message]

ticket: 7476
tags: pullup
target_version: 1.18-next