uefi-services: Return event in init

As mentioned in #917
If the app built with uefi-services exits before ExitBootServices, UEFI
will call `UnloadImage` on it. Then the callback function is not in
memory anymore and when ExitBootServices runs, invalid memory is
accessed.

QEMU will crash like this when the Linux EFISTUB calls ExitBootServices:

```
!!!! X64 Exception Type - 06(#UD - Invalid Opcode)  CPU Apic ID - 00000000 !!!!
RIP  - 000000003E13F018, CS  - 0000000000000038, RFLAGS - 0000000000210246
RAX  - 0000000000000000, RCX - 000000003E191E98, RDX - 0000000000000000
RBX  - 0000000000000004, RSP - 000000003FF0FD08, RBP - 000000003FF28740
RSI  - 000000003E167518, RDI - 000000003FF6A5E0
R8   - 000000003FF27140, R9  - 000000003F4807A0, R10 - 0000000000000001
R11  - 00000000000000E1, R12 - 000000003E191EE0, R13 - 0000000000000100
R14  - 000000003FF28840, R15 - 000000003E191E98
DS   - 0000000000000030, ES  - 0000000000000030, FS  - 0000000000000030
GS   - 0000000000000030, SS  - 0000000000000030
CR0  - 0000000080010033, CR2 - 0000000000000000, CR3 - 000000003FC01000
CR4  - 0000000000000668, CR8 - 0000000000000000
DR0  - 0000000000000000, DR1 - 0000000000000000, DR2 - 0000000000000000
DR3  - 0000000000000000, DR6 - 00000000FFFF0FF0, DR7 - 0000000000000400
GDTR - 000000003F9DE000 0000000000000047, LDTR - 0000000000000000
IDTR - 000000003F471018 0000000000000FFF,   TR - 0000000000000000
FXSAVE_STATE - 000000003FF0F960
!!!! Find image based on IP(0x3E13F018) (No PDB)  (ImageBase=000000003DF1E000, EntryPoint=000000003DF1F000) !!!!
```

To avoid the crash, the user should close the event before the app
exits.
```rs
fn main(image_handle: Handle, mut system_table: SystemTable<Boot>) -> Status {
    let event: Option<Event> = uefi_services::init(&mut system_table).unwrap();
    let boot_services = system_table.boot_services();

    println!("Hello World");

    if let Some(event) = event {
        // Will creash when Linux boots if we don't call this
        boot_services.close_event(event).unwrap();
    }
    Status::SUCCESS
}
```

It would be nice to have this happen automatically by a Drop
implementation but this already lets users do it on their own, if they
need.

Signed-off-by: Daniel Schaefer <dhs@frame.work>

Author: JohnAZoidberg

uefi-services/src/lib.rs

@@ -78,11 +78,11 @@ pub fn system_table() -> NonNull<SystemTable<Boot>> {
 ///
 /// This must be called as early as possible,
 /// before trying to use logging or memory allocation capabilities.
-pub fn init(st: &mut SystemTable<Boot>) -> Result {
+pub fn init(st: &mut SystemTable<Boot>) -> Result<Option<Event>> {
     unsafe {
         // Avoid double initialization.
         if SYSTEM_TABLE.is_some() {
-            return Status::SUCCESS.to_result();
+            return Status::SUCCESS.to_result_with_val(|| None);
         }
 
         // Setup the system table singleton
@@ -104,7 +104,7 @@ pub fn init(st: &mut SystemTable<Boot>) -> Result {
                 Some(exit_boot_services),
                 None,
             )
-            .map(|_| ())
+            .map(|e| Some(e))
     }
 }