cmd/omitsize: add flag to disable the removal table

And remove a bogus omit feature from feature/featuretags.

Updates #12614

Change-Id: I0a08183fb75c73ae75b6fd4216d134e352dcf5a0
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

feature/featuretags/featuretags.go

@@ -4,7 +4,37 @@
 // The featuretags package is a registry of all the ts_omit-able build tags.
 package featuretags
 
-var Features = map[string]string{
+// CLI is a special feature in the [Features] map that works opposite
+// from the others: it is opt-in, rather than opt-out, having a different
+// build tag format.
+const CLI FeatureTag = "cli"
+
+// FeatureTag names a Tailscale feature that can be selectively added or removed
+// via build tags.
+type FeatureTag string
+
+// IsOmittable reports whether this feature tag is one that can be
+// omitted via a ts_omit_ build tag.
+func (ft FeatureTag) IsOmittable() bool {
+	switch ft {
+	case CLI:
+		return false
+	}
+	return true
+}
+
+// OmitTag returns the ts_omit_ build tag for this feature tag.
+// It panics if the feature tag is not omitable.
+func (ft FeatureTag) OmitTag() string {
+	if !ft.IsOmittable() {
+		panic("not omitable: " + string(ft))
+	}
+	return "ts_omit_" + string(ft)
+}
+
+// Features are the known Tailscale features that can be selectively included or
+// excluded via build tags, and a description of each.
+var Features = map[FeatureTag]string{
 	"aws":              "AWS integration",
 	"bird":             "Bird BGP integration",
 	"capture":          "Packet capture",
@@ -21,7 +51,6 @@ var Features = map[string]string{
 	"taildrop":         "Taildrop (file sending) support",
 	"tailnetlock":      "Tailnet Lock support",
 	"tap":              "Experimental Layer 2 (ethernet) support",
-	"tka":              "Tailnet Lock (TKA) support",
 	"tpm":              "TPM support",
 	"wakeonlan":        "Wake-on-LAN support",
 	"webclient":        "Web client support",