cmd/v: rename reuse-tmpc to keepc

cmd/tools/vcomplete.v

@@ -122,7 +122,7 @@ const (
 		'-parallel',
 		'-x64',
 		'-W',
-		'-reuse-tmpc',
+		'-keepc',
 		'-w',
 		'-print_v_files',
 		'-error-limit',

cmd/v/help/build-c.txt

@@ -59,5 +59,5 @@ These build flags are enabled on `build` and `run` as long as the backend is set
       Useful for similar use-case as above option, except it replaces V-generated prelude with
       your custom one loaded from specified <path>.
 
-   -reuse-tmpc
+   -keepc
       Do not remove the temporary .tmp.c and .tmp.c.rsp files. Also do not use a random prefix for them, so they would be fixed and predictable.

vlib/v/pref/pref.v

@@ -268,7 +268,7 @@ pub fn parse_args(args []string) (&Preferences, string) {
 			'-W' {
 				res.warns_are_errors = true
 			}
-			'-reuse-tmpc' {
+			'-keepc' {
 				res.reuse_tmpc = true
 			}
 			'-w' {