Test FlagHasPid with != 0, not == 1

Masking with a flag constant then comparing to 1 only works while the
flag is bit 0; != 0 is correct for any bit position and matches every
other flag check in the codebase.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: markmnl

cmd/fmsgd/host.go

@@ -1018,7 +1018,7 @@ func readHeader(c net.Conn) (*FMsgHeader, *bufio.Reader, error) {
 	}
 
 	// read pid if any
-	if flags&FlagHasPid == 1 {
+	if flags&FlagHasPid != 0 {
 		pid, err := io.ReadAll(io.LimitReader(r, 32))
 		if err != nil {
 			return h, r, err

pkg/fmsg/fmsg.go

@@ -90,7 +90,7 @@ func (h *Header) Encode() []byte {
 	var b bytes.Buffer
 	b.WriteByte(h.Version)
 	b.WriteByte(h.Flags)
-	if h.Flags&FlagHasPid == 1 {
+	if h.Flags&FlagHasPid != 0 {
 		b.Write(h.Pid[:])
 	}
 	str := h.From.ToString()