envknob,net/tstun,wgengine: use TS_DEBUG_MTU consistently

Noted on #5915 TS_DEBUG_MTU was not used consistently everywhere.
Extract the default into a function that can apply this centrally and
use it everywhere.

Added envknob.Lookup{Int,Uint}Sized to make it easier to keep CodeQL
happy when using converted values.

Updates #5915

Signed-off-by: James Tucker <james@tailscale.com>

envknob/envknob.go

@@ -330,6 +330,46 @@ func LookupInt(envVar string) (v int, ok bool) {
 	panic("unreachable")
 }
 
+// LookupIntSized returns the integer value of the named environment value
+// parsed in base and with a maximum bit size bitSize.
+// The ok result is whether a value was set.
+// If the value isn't a valid int, it exits the program with a failure.
+func LookupIntSized(envVar string, base, bitSize int) (v int, ok bool) {
+	assertNotInInit()
+	val := os.Getenv(envVar)
+	if val == "" {
+		return 0, false
+	}
+	i, err := strconv.ParseInt(val, base, bitSize)
+	if err == nil {
+		v = int(i)
+		noteEnv(envVar, val)
+		return v, true
+	}
+	log.Fatalf("invalid integer environment variable %s: %v", envVar, val)
+	panic("unreachable")
+}
+
+// LookupUintSized returns the unsigned integer value of the named environment
+// value parsed in base and with a maximum bit size bitSize.
+// The ok result is whether a value was set.
+// If the value isn't a valid int, it exits the program with a failure.
+func LookupUintSized(envVar string, base, bitSize int) (v uint, ok bool) {
+	assertNotInInit()
+	val := os.Getenv(envVar)
+	if val == "" {
+		return 0, false
+	}
+	i, err := strconv.ParseUint(val, base, bitSize)
+	if err == nil {
+		v = uint(i)
+		noteEnv(envVar, val)
+		return v, true
+	}
+	log.Fatalf("invalid unsigned integer environment variable %s: %v", envVar, val)
+	panic("unreachable")
+}
+
 // UseWIPCode is whether TAILSCALE_USE_WIP_CODE is set to permit use
 // of Work-In-Progress code.
 func UseWIPCode() bool { return Bool("TAILSCALE_USE_WIP_CODE") }