pkg/mount: implement/use filter for mountinfo parsing

Functions `GetMounts()` and `parseMountTable()` return all the entries
as read and parsed from /proc/self/mountinfo. In many cases the caller
is only interested only one or a few entries, not all of them.

One good example is `Mounted()` function, which looks for a specific
entry only. Another example is `RecursiveUnmount()` which is only
interested in mount under a specific path.

This commit adds `filter` argument to `GetMounts()` to implement
two things:
 1. filter out entries a caller is not interested in
 2. stop processing if a caller is found what it wanted

`nil` can be passed to get a backward-compatible behavior, i.e. return
all the entries.

A few filters are implemented:
 - `PrefixFilter`: filters out all entries not under `prefix`
 - `SingleEntryFilter`: looks for a specific entry

Finally, `Mounted()` is modified to use `SingleEntryFilter()`, and
`RecursiveUnmount()` is using `PrefixFilter()`.

Unit tests are added to check filters are working.

[v2: ditch NoFilter, use nil]
[v3: ditch GetMountsFiltered()]
[v4: add unit test for filters]
[v5: switch to gotestyourself]

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

integration-cli/docker_cli_daemon_plugins_test.go

@@ -260,7 +260,7 @@ func (s *DockerDaemonSuite) TestPluginVolumeRemoveOnRestart(c *check.C) {
 }
 
 func existsMountpointWithPrefix(mountpointPrefix string) (bool, error) {
-	mounts, err := mount.GetMounts()
+	mounts, err := mount.GetMounts(nil)
 	if err != nil {
 		return false, err
 	}