improve header searchlist handling in build, main

build.zig:
- use "-I" instead of "-isystem" for `b.addSearchPrefix()`

main.zig:
- silently ignore superfluous search dirs
- warn when a dir is added to multiple searchlists
- consolidate "expected paramter after {s}" fatal error messages
- rename command-line switch `-dirafter` → `-idirafter`

closes #12888

lib/std/build.zig

@@ -3123,7 +3123,7 @@ pub const LibExeObjStep = struct {
             try zig_args.append(builder.pathJoin(&.{
                 search_prefix, "lib",
             }));
-            try zig_args.append("-isystem");
+            try zig_args.append("-I");
             try zig_args.append(builder.pathJoin(&.{
                 search_prefix, "include",
             }));