Feature user for emscripten

[ChanTsune]

What does this PR do

Fix failed to build for target wasm32-unknown-emscripten with --features user with this error:

error[E0432]: unresolved import `self::os`
 --> src/features.rs:2:15
  |
2 | pub use self::os::*;
  |               ^^ could not find `os` in `self`

error[E0609]: no field `pw_class` on type `&passwd`
    --> src/unistd.rs:3569:55
     |
3569 |                 class: CString::new(CStr::from_ptr(pw.pw_class).to_bytes())
     |                                                       ^^^^^^^^ unknown field
     |
     = note: available fields are: `pw_name`, `pw_passwd`, `pw_uid`, `pw_gid`, `pw_gecos` ... and 2 others

error[E0609]: no field `pw_change` on type `&passwd`
    --> src/unistd.rs:3579:28
     |
3579 |                 change: pw.pw_change,
     |                            ^^^^^^^^^ unknown field
     |
     = note: available fields are: `pw_name`, `pw_passwd`, `pw_uid`, `pw_gid`, `pw_gecos` ... and 2 others

error[E0609]: no field `pw_expire` on type `&passwd`
    --> src/unistd.rs:3588:28
     |
3588 |                 expire: pw.pw_expire,
     |                            ^^^^^^^^^ unknown field
     |
     = note: available fields are: `pw_name`, `pw_passwd`, `pw_uid`, `pw_gid`, `pw_gecos` ... and 2 others

error[E0560]: struct `passwd` has no field named `pw_class`
    --> src/unistd.rs:3629:13
     |
3629 |             pw_class: u.class.into_raw(),
     |             ^^^^^^^^ `passwd` does not have this field
     |
     = note: all struct fields are already assigned

error[E0560]: struct `passwd` has no field named `pw_change`
    --> src/unistd.rs:3638:13
     |
3638 |             pw_change: u.change,
     |             ^^^^^^^^^ `passwd` does not have this field
     |
     = note: all struct fields are already assigned

error[E0560]: struct `passwd` has no field named `pw_expire`
    --> src/unistd.rs:3647:13
     |
3647 |             pw_expire: u.expire,
     |             ^^^^^^^^^ `passwd` does not have this field
     |
     = note: all struct fields are already assigned

Some errors have detailed explanations: E0432, E0560, E0609.
For more information about an error, try `rustc --explain E0432`.
error: could not compile `nix` (lib) due to 7 previous errors

• struct passwd for emscripten not have some fields, so disabled it.

Even if this error is resolved, full support for emscripten will not be possible because libc does not provide some functions for emscripten like libc::getpwuid_r. but this change will be useful in the future.

Checklist:

• I have read CONTRIBUTING.md
• I have written necessary tests and rustdoc comments
• A change log has been added if this PR modifies nix's API

[SteveLauC]

LGTM, thanks for the patch!