synced_folders/rsync: SSH control for faster rsync

[mitchellh]

Fixes #6399

This uses OpenSSH control paths for a faster rsync. On my system where rsync was already fairly fast, it went from ~250ms on every invocation to ~175ms. I've heard on some systems executing rsync is hundreds more ms, so this should speed up even more.

[mkuzmin]

This change broke synchronization from Windows hosts.

C:\Users\michael.kuzmin\Projects\forks\vagrant>bundle exec vagrant rsync
==> teamcity: Rsyncing folder: /c/Users/michael.kuzmin/Projects/forks/vagrant/tasks/ => /vagrant
There was an error when attempting to rsync a synced folder.
Please inspect the error message below for more info.

Host path: /c/Users/michael.kuzmin/Projects/forks/vagrant/tasks/
Guest path: /vagrant
Command: rsync --verbose --archive --delete -z --copy-links --chmod=ugo=rwX --no-perms --no-owner --no-group --rsync-path
sudo rsync -e ssh -p 22 -o ControlMaster=auto -o ControlPath=C:/Users/MICHAE~1.KUZ/AppData/Local/Temp/ssh.386 -o ControlPersist=10m -o StrictHostKeyChecking=no -o IdentitiesOnly=true -o UserKnownHostsFile=/dev/null -i 'C:/Users/michael.kuzmin/Projects/forks/vagrant/.vagrant/machines/teamcity/vsphere/private_key' --exclude .vagrant/ /c/Users/michael.kuzmin/Projects/forks/vagrant/tasks/ jetbrains@172.20.160.27:/vagrant

Warning: Permanently added '172.20.160.27' (ECDSA) to the list of known hosts.
mm_receive_fd: no message header
process_mux_new_session: failed to receive fd 0 from slave
mux_client_request_session: read from master failed: Connection reset by peer
Failed to connect to new control master
rsync: connection unexpectedly closed (0 bytes received so far) [sender]
rsync error: error in rsync protocol data stream (code 12) at io.c(226) [sender=3.1.1]

ControlPath is resolved to C:/Users/MICHAE~1.KUZ/AppData/Local/Temp/ssh.386 instead of /c/Users/michael.kuzmin/AppData/Local/Temp/ssh.386

[mkuzmin]

It's a kind of bug in Windows, but that's a common behavior in several Windows versions.
Dir.tmpdir() is resolved to %TEMP%, TEMP depends from USERPROFILE

If username is longer than 8 symbols, Windows resolves it to such value in Windows 95 style.

> set
TEMP=C:\Users\MICHAE~1.KUZ\AppData\Local\Temp
TMP=C:\Users\MICHAE~1.KUZ\AppData\Local\Temp
USERPROFILE=C:\Users\michael.kuzmin