japanese/skkinput: Remove invalid option

Remove RSKKSERV option (Depends on japanese/rskkserv expired in 2014-09)
Add LICENSE_FILE
Add japanese/skkserv option
If not selected in the options, do not install dictionary server
Switch post-install to post-install-DOCS-on

PR: 279374
Approved by: koma2@lovepeers.org (maintainer, timeout > 1 month)