net/dns/resolver: handle tabs as whitespace when ExitDNS parses resolv.conf

On Synology, the /etc/resolv.conf has tabs in it, which this
resolv.conf parser (we have two, sigh) didn't handle.

Updates #3710

Change-Id: I86f8e09ad1867ee32fa211e85c382a27191418ea
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
(cherry picked from commit 24a04d07d1)

net/dns/resolver/tsdns.go

@@ -529,6 +529,10 @@ func stubResolverForOS() (ip netaddr.IP, err error) {
 	if c, ok := resolvConfCacheValue.Load().(resolvConfCache); ok && c.mod == cur.mod && c.size == cur.size {
 		return c.ip, nil
 	}
+	// TODO(bradfitz): unify this /etc/resolv.conf parsing code with readResolv
+	// in net/dns, which we can't use due to circular dependency reasons.
+	// Move it to a leaf, including the OSConfig type (perhaps in its own dnstype
+	// package?)
 	err = lineread.File("/etc/resolv.conf", func(line []byte) error {
 		if !ip.IsZero() {
 			return nil
@@ -537,6 +541,12 @@ func stubResolverForOS() (ip netaddr.IP, err error) {
 		if len(line) == 0 || line[0] == '#' {
 			return nil
 		}
+		// Normalize tabs to spaces to simplify parsing code later.
+		for i, b := range line {
+			if b == '\t' {
+				line[i] = ' '
+			}
+		}
 		if mem.HasPrefix(mem.B(line), mem.S("nameserver ")) {
 			s := strings.TrimSpace(strings.TrimPrefix(string(line), "nameserver "))
 			ip, err = netaddr.ParseIP(s)