tailfs: initial implementation

Add a WebDAV-based folder sharing mechanism that is exposed to local clients at
100.100.100.100:8080 and to remote peers via a new peerapi endpoint at
/v0/tailfs.

Add the ability to manage folder sharing via the new 'share' CLI sub-command.

Updates tailscale/corp#16827

Signed-off-by: Percy Wegmann <percy@tailscale.com>

tstest/integration/tailscaled_deps_test_darwin.go

@@ -38,6 +38,7 @@ import (
 	_ "tailscale.com/ssh/tailssh"
 	_ "tailscale.com/syncs"
 	_ "tailscale.com/tailcfg"
+	_ "tailscale.com/tailfs"
 	_ "tailscale.com/tsd"
 	_ "tailscale.com/tsweb/varz"
 	_ "tailscale.com/types/flagtype"

tstest/integration/tailscaled_deps_test_freebsd.go

@@ -38,6 +38,7 @@ import (
 	_ "tailscale.com/ssh/tailssh"
 	_ "tailscale.com/syncs"
 	_ "tailscale.com/tailcfg"
+	_ "tailscale.com/tailfs"
 	_ "tailscale.com/tsd"
 	_ "tailscale.com/tsweb/varz"
 	_ "tailscale.com/types/flagtype"

tstest/integration/tailscaled_deps_test_linux.go

@@ -38,6 +38,7 @@ import (
 	_ "tailscale.com/ssh/tailssh"
 	_ "tailscale.com/syncs"
 	_ "tailscale.com/tailcfg"
+	_ "tailscale.com/tailfs"
 	_ "tailscale.com/tsd"
 	_ "tailscale.com/tsweb/varz"
 	_ "tailscale.com/types/flagtype"

tstest/integration/tailscaled_deps_test_openbsd.go

@@ -38,6 +38,7 @@ import (
 	_ "tailscale.com/ssh/tailssh"
 	_ "tailscale.com/syncs"
 	_ "tailscale.com/tailcfg"
+	_ "tailscale.com/tailfs"
 	_ "tailscale.com/tsd"
 	_ "tailscale.com/tsweb/varz"
 	_ "tailscale.com/types/flagtype"

tstest/integration/tailscaled_deps_test_windows.go

@@ -45,6 +45,7 @@ import (
 	_ "tailscale.com/safesocket"
 	_ "tailscale.com/syncs"
 	_ "tailscale.com/tailcfg"
+	_ "tailscale.com/tailfs"
 	_ "tailscale.com/tsd"
 	_ "tailscale.com/tsweb/varz"
 	_ "tailscale.com/types/flagtype"