* add CUD support for channel members via plugins
This effectively exposes AddChannelMember, UpdateChannelMemberRoles,
UpdateChannelMemberNotifyProps and LeaveChannel via the plugin API.
It also modifies the semantics of AddChannelMember to explicitly allow
for an empty user requestor, left as such for now via the plugin API.
* change the signature of AddChannelMember to accept a channel id instead of a channel
* plugin sandboxing
* remove unused type
* better symlink handling, better remounting, better test, whitespace
fixes, and comment on the remounting
* fix test compile error
* big simplification for getting mount flags
* mask statfs flags to the ones we're interested in
* tie back-end plugins together
* fix comment typo
* add tests and a bit of polish
* tests and polish
* add test, don't let backend executable paths escape the plugin directory
* Implement experimental REST API endpoints for plugins
* Updates per feedback and rebase
* Update tests
* Further updates
* Update extraction of plugins
* Use OS temp dir for plugins instead of search path
* Fail extraction on paths that attempt to traverse upward
* Update pluginenv ActivePlugins()
* begin backend plugin wip
* flesh out rpcplugin. everything done except for minor supervisor stubs
* done with basic plugin infrastructure
* simplify tests
* remove unused test lines
