all: support v -watch run (#9577)

vlib/os/fd.c.v

@@ -4,10 +4,16 @@ module os
 
 // close filedescriptor
 pub fn fd_close(fd int) int {
+	if fd == -1 {
+		return 0
+	}
 	return C.close(fd)
 }
 
 pub fn fd_write(fd int, s string) {
+	if fd == -1 {
+		return
+	}
 	mut sp := s.str
 	mut remaining := s.len
 	for remaining > 0 {
@@ -23,6 +29,9 @@ pub fn fd_write(fd int, s string) {
 // read from filedescriptor, block until data
 pub fn fd_slurp(fd int) []string {
 	mut res := []string{}
+	if fd == -1 {
+		return res
+	}
 	for {
 		s, b := fd_read(fd, 4096)
 		if b <= 0 {
@@ -36,6 +45,9 @@ pub fn fd_slurp(fd int) []string {
 // read from filedescriptor, don't block
 // return [bytestring,nrbytes]
 pub fn fd_read(fd int, maxbytes int) (string, int) {
+	if fd == -1 {
+		return '', 0
+	}
 	unsafe {
 		mut buf := malloc(maxbytes)
 		nbytes := C.read(fd, buf, maxbytes)