#112 followup (bugfix and fix build)

[Lurk]

This is #112 follow-up with two changes:

1. We missed this part: "It is not possible to block SIGKILL or SIGSTOP. Attempts to do so are silently ignored." (https://man7.org/linux/man-pages/man2/sigprocmask.2.html) addressing that
2. Fixing Clippy issue:

error: redundant pattern matching, consider using `is_err()`
  --> kernel/syscalls/rt_sigprocmask.rs:27:16
   |
27 |         if let Err(_) = current_process().set_signal_mask(how, set, oldset, length) {
   |         -------^^^^^^-------------------------------------------------------------- help: try this: `if current_process().set_signal_mask(how, set, oldset, length).is_err()`
   |
   = note: `-D clippy::redundant-pattern-matching` implied by `-D warnings`

which we introduced in #112

[nuta]

Looks good. Thanks!