all: improve unused variable warning (fix x = 1, x += 1 etc)

vlib/os/os_c.v

@@ -740,11 +740,10 @@ pub fn is_link(path string) bool {
 
 // chdir changes the current working directory to the new directory in `path`.
 pub fn chdir(path string) {
-	mut n := 0
 	$if windows {
 		C._wchdir(path.to_wide())
 	} $else {
-		n = C.chdir(&char(path.str))
+		_ = C.chdir(&char(path.str))
 	}
 }
 

vlib/os/process_nix.c.v

@@ -3,12 +3,11 @@ module os
 fn C.setpgid(pid int, pgid int) int
 
 fn (mut p Process) unix_spawn_process() int {
-	mut n := 0
 	mut pipeset := [6]int{}
 	if p.use_stdio_ctl {
-		n = C.pipe(&pipeset[0]) // pipe read end 0 <- 1 pipe write end
-		n = C.pipe(&pipeset[2]) // pipe read end 2 <- 3 pipe write end
-		n = C.pipe(&pipeset[4]) // pipe read end 4 <- 5 pipe write end
+		_ = C.pipe(&pipeset[0]) // pipe read end 0 <- 1 pipe write end
+		_ = C.pipe(&pipeset[2]) // pipe read end 2 <- 3 pipe write end
+		_ = C.pipe(&pipeset[4]) // pipe read end 4 <- 5 pipe write end
 	}
 	pid := fork()
 	if pid != 0 {