Fix for 0.10.0

[tekktonic]

Most changes performed by zig fmt. The only manual changes I made were as follows:

1. there were a few instances zig fmt refused to fix for some reason of operators not being surrounded by whitespace (request.zig:880, datetime.zig:1528.)
2. anytype is no longer allowed as the type for a field, you have to use the equivalent ?*const anyopaque (see eliminate anytype fields from the language ziglang/zig#10705)
3. OpenFlags now uses an enum for modes rather than just true/false (https://github.com/ziglang/zig/blob/master/lib/std/fs/file.zig#L80) this affected mimetypes.zig:281 and handlers.zig:113
4. os.sigaction can now return an error, so put a try on it in app.zig:629

[frmdstryr]

Works here. Thanks!

[YouJiacheng]

?*const anyopaque is not an equivalent of anytype, just an alternative. Its usage is different.
see https://github.com/ziglang/zig/pull/10766/files#diff-810d7757edd41ea303795cbc7b7e2a6f1a9ab93fbd91db1495796e791997811e