libzigc: stropts/isastream

`isastream` is an old, deprecated function for the STREAMS framework.
Linux doesn't even support this natively, and so the musl implementation
just returns 0 (not a STREAMS file) for valid file descriptors or -1
(for bad file descriptors).

lib/c.zig

@@ -73,6 +73,7 @@ comptime {
     _ = @import("c/stdlib.zig");
     _ = @import("c/string.zig");
     _ = @import("c/strings.zig");
+    _ = @import("c/stropts.zig");
 
     _ = @import("c/sys/capability.zig");
     _ = @import("c/sys/file.zig");

lib/c/stropts.zig

@@ -0,0 +1,17 @@
+const builtin = @import("builtin");
+
+const std = @import("std");
+const linux = std.os.linux;
+
+const symbol = @import("../c.zig").symbol;
+const errno = @import("../c.zig").errno;
+
+comptime {
+    if (builtin.target.isMuslLibC()) {
+        symbol(&isastream, "isastream");
+    }
+}
+
+fn isastream(fd: c_int) callconv(.c) c_int {
+    return if (errno(linux.fcntl(fd, linux.F.GETFD, 0)) < 0) -1 else 0;
+}

lib/libc/musl/src/legacy/isastream.c

@@ -1,7 +0,0 @@
-#include <stropts.h>
-#include <fcntl.h>
-
-int isastream(int fd)
-{
-	return fcntl(fd, F_GETFD) < 0 ? -1 : 0;
-}

src/libs/musl.zig

@@ -691,7 +691,6 @@ const src_files = [_][]const u8{
     "musl/src/legacy/getpagesize.c",
     "musl/src/legacy/getpass.c",
     "musl/src/legacy/getusershell.c",
-    "musl/src/legacy/isastream.c",
     "musl/src/legacy/lutimes.c",
     "musl/src/legacy/ulimit.c",
     "musl/src/legacy/utmpx.c",