Fix crash on NetBSD

The `_lwp_self` function cannot be used to reliably determine the main
thread, see
https://github.com/alacritty/alacritty/issues/2631#issuecomment-676723289.

It might always be equal to the PID, but it's certainly not always 1
when the thread is the main thread.

However, Rust's built in `Thread::id` and `Thread::name` function will
always return `ThreadId(1)` and `Some("main")`. Since converting the
thread's ID to a number is not supported on stable Rust, checking that
the thread is labeled `Some("main")` seems like the most reliable
option. It should also be a good fallback in general.

src/platform_impl/linux/mod.rs

@@ -730,7 +730,5 @@ fn is_main_thread() -> bool {
 
 #[cfg(target_os = "netbsd")]
 fn is_main_thread() -> bool {
-    use libc::_lwp_self;
-
-    unsafe { _lwp_self() == 1 }
+    std::thread::current().name() == Some("main")
 }