stdlib: Use "RelativePath" struct for `*at()` POSIX functions in `std.os`

[topolarity]

Publishing this at @kubkon 's recommendation in #11021 (comment)

That PR introduced std.os.RelativePathWasi, which looks like this:

/// An fd-relative file path
pub const RelativePathWasi = struct {
    /// Handle to directory
    dir_fd: fd_t,
    /// Path to resource within `dir_fd`.
    relative_path: []const u8,
};

Although this only used in the WASI-specific internal functions right now, it seems natural for all of the (slightly) more modern *at() POSIX functions: openat, renameat, symlinkat, etc.

The idea would be to change these functions to accept RelativePath for fd_t-relative path arguments:

// Proposed
fn openat(file: RelativePath, flags: u32, mode: mode_t) OpenError!fd_t
fn symlinkat(target: []const u8, symlink: RelativePath) SymLinkError!void
fn renameat(old: RelativePath, new: RelativePath) RenameError!void

// Current
fn openat(dir_fd: fd_t, file_path: []const u8, flags: u32, mode: mode_t) OpenError!fd_t
fn symlinkat(target_path: []const u8, newdirfd: fd_t, sym_link_path: []const u8) SymLinkError!void
fn renameat(old_dir_fd: fd_t, old_path: []const u8, new_dir_fd: fd_t, new_path: []const u8) RenameError!void

This comes with some other small advantages:

• POSIX-style functions can be made to be generic over fd_t/[]const u8/RelativePath parameters, if desired
• It matches the cwd().openFile() style in the "std.fs" interface well - e.g. cwd().getRelativePath(...) could give you a RelativePath for use with the POSIX-style std.os functions