v2.1.1: Remount After Network Drop

Fixes

• :SSHConnect to a mount that died, for example after a network drop, now replaces the dead mount. Before, it only warned "already mounted", and a hung mount froze Neovim. (#39)
• The shared SSH connection now gets the ServerAlive* values from sshfs_options. It exits within about 45 seconds of a drop, so sshfs reconnect can recover the mount without help. This needs key auth.
• ServerAlive* keys in sshfs_options work in any case, and a key such as serveraliveinterval replaces the default.
• Closing the shared SSH connection times out after 5 seconds, instead of blocking Neovim when the connection hangs.

Full Changelog: v2.1.0...v2.1.1