libct: speedup process.Env handling

The current implementation sets all the environment variables passed in
Process.Env in the current process, one by one, then uses os.Environ to
read those back.

As pointed out in [1], this is slow, as runc calls os.Setenv for every
variable, and there may be a few thousands of those. Looking into how
os.Setenv is implemented, it is indeed slow, especially when cgo is
enabled.

Looking into why it was implemented the way it is, I found commit
9744d72c and traced it to [2], which discusses the actual reasons.
It boils down to these two:

 - HOME is not passed into container as it is set in setupUser by
   os.Setenv and has no effect on config.Env;
 - there is a need to deduplicate the environment variables.

Yet it was decided in [2] to not go ahead with this patch, but
later [3] was opened with the carry of this patch, and merged.

Now, from what I see:

1. Passing environment to exec is way faster than using os.Setenv and
   os.Environ (tests show ~20x speed improvement in a simple Go test,
   and ~3x improvement in real-world test, see below).
2. Setting environment variables in the runc context may result is some
   ugly side effects (think GODEBUG, LD_PRELOAD, or _LIBCONTAINER_*).
3. Nothing in runtime spec says that the environment needs to be
   deduplicated, or the order of preference (whether the first or the
   last value of a variable with the same name is to be used). We should
   stick to what we have in order to maintain backward compatibility.

So, this patch:
 - switches to passing env directly to exec;
 - adds deduplication mechanism to retain backward compatibility;
 - takes care to set PATH from process.Env in the current process
   (so that supplied PATH is used to find the binary to execute),
   also to retain backward compatibility;
 - adds HOME to process.Env if not set;
 - ensures any StartContainer CommandHook entries with no environment
   set explicitly are run with the same environment as before. Thanks
   to @lifubang who noticed that peculiarity.

The benchmark added by the previous commit shows ~3x improvement:

	                │   before    │                after                 │
	                │   sec/op    │    sec/op     vs base                │
	ExecInBigEnv-20   61.53m ± 1%   21.87m ± 16%  -64.46% (p=0.000 n=10)

[1]: https://github.com/opencontainers/runc/pull/1983
[2]: https://github.com/docker-archive/libcontainer/pull/418
[3]: https://github.com/docker-archive/libcontainer/pull/432

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

libcontainer/standard_init_linux.go

@@ -27,6 +27,7 @@ type linuxStandardInit struct {
 	fifoFile      *os.File
 	logPipe       *os.File
 	config        *initConfig
+	addHome       bool
 }
 
 func (l *linuxStandardInit) getSessionRingParams() (string, uint32, uint32) {
@@ -186,7 +187,7 @@ func (l *linuxStandardInit) Init() error {
 			return err
 		}
 	}
-	if err := finalizeNamespace(l.config); err != nil {
+	if err := finalizeNamespace(l.config, l.addHome); err != nil {
 		return err
 	}
 	// finalizeNamespace can change user/group which clears the parent death
@@ -194,6 +195,17 @@ func (l *linuxStandardInit) Init() error {
 	if err := pdeath.Restore(); err != nil {
 		return fmt.Errorf("can't restore pdeath signal: %w", err)
 	}
+
+	// In case we have any StartContainer hooks to run, and they don't
+	// have environment configured explicitly, make sure they will be run
+	// with the same environment as container's init.
+	//
+	// NOTE the above described behavior is not part of runtime-spec, but
+	// rather a de facto historical thing we afraid to change.
+	if h := l.config.Config.Hooks[configs.StartContainer]; len(h) > 0 {
+		h.SetDefaultEnv(l.config.Env)
+	}
+
 	// Compare the parent from the initial start of the init process and make
 	// sure that it did not change.  if the parent changes that means it died
 	// and we were reparented to something else so we should just kill ourself
@@ -285,5 +297,5 @@ func (l *linuxStandardInit) Init() error {
 	if err := utils.UnsafeCloseFrom(l.config.PassedFilesCount + 3); err != nil {
 		return err
 	}
-	return system.Exec(name, l.config.Args, os.Environ())
+	return system.Exec(name, l.config.Args, l.config.Env)
 }