cmd/tailscaled, logpolicy, logtail: support log levels

Log levels can now be specified with "[v1] " or "[v2] " substrings
that are then stripped and filtered at the final logger. This follows
our existing "[unexpected]" etc convention and doesn't require a
wholesale reworking of our logging at the moment.

cmd/tailscaled then gets a new --verbose=N flag to take a log level
that controls what gets logged to stderr (and thus systemd, syslog,
etc). Logtail is unaffected by --verbose.

This commit doesn't add annotations to any existing log prints. That
is in the next commit.

Updates #924
Updates #282

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

logpolicy/logpolicy.go

@@ -413,6 +413,15 @@ func New(collection string) *Policy {
 	}
 }
 
+// SetVerbosityLevel controls the verbosity level that should be
+// written to stderr. 0 is the default (not verbose). Levels 1 or higher
+// are increasingly verbose.
+//
+// It should not be changed concurrently with log writes.
+func (p *Policy) SetVerbosityLevel(level int) {
+	p.Logtail.SetVerbosityLevel(level)
+}
+
 // Close immediately shuts down the logger.
 func (p *Policy) Close() {
 	ctx, cancel := context.WithCancel(context.Background())