std/os/uefi: Simplify packed struct padding and default zero-initialize

Beyond adding default zero-initialization, this commit changes undefined
initialization to zero, as some cases reserved the padding and on other
cases, I've found some systems act strange when giving uninit instead of
zero even when it shouldn't be an issue, one example being
FileProtocol.Open's attributes, which *should* be ignored when not
creating a file, but ended up giving an unrelated error.

lib/std/os/uefi.zig

@@ -98,7 +98,6 @@ pub const Time = extern struct {
 
     /// 0 - 59
     second: u8,
-    _pad1: u8,
 
     /// 0 - 999999999
     nanosecond: u32,
@@ -115,7 +114,6 @@ pub const Time = extern struct {
         /// If true, the time is affected by daylight savings time.
         adjust_daylight: bool,
     },
-    _pad2: u8,
 
     /// Time is to be interpreted as local time
     pub const unspecified_timezone: i16 = 0x7ff;

lib/std/os/uefi/protocols/absolute_pointer_protocol.zig

@@ -40,9 +40,7 @@ pub const AbsolutePointerMode = extern struct {
     attributes: packed struct {
         supports_alt_active: bool,
         supports_pressure_as_z: bool,
-        _pad1: u6,
-        _pad2: u8,
-        _pad3: u16,
+        _pad: u30 = 0,
     },
 };
 
@@ -53,8 +51,6 @@ pub const AbsolutePointerState = extern struct {
     active_buttons: packed struct {
         touch_active: bool,
         alt_active: bool,
-        _pad1: u6,
-        _pad2: u8,
-        _pad3: u16,
+        _pad: u30 = 0,
     },
 };

lib/std/os/uefi/protocols/edid_override_protocol.zig

@@ -26,7 +26,5 @@ pub const EdidOverrideProtocol = extern struct {
 pub const EdidOverrideProtocolAttributes = packed struct {
     dont_override: bool,
     enable_hot_plug: bool,
-    _pad1: u6,
-    _pad2: u8,
-    _pad3: u16,
+    _pad: u30 = 0,
 };

lib/std/os/uefi/protocols/hii.zig

@@ -48,7 +48,7 @@ pub const NarrowGlyph = extern struct {
     attributes: packed struct {
         non_spacing: bool,
         wide: bool,
-        _pad: u6,
+        _pad: u6 = 0,
     },
     glyph_col_1: [19]u8,
 };
@@ -62,7 +62,7 @@ pub const WideGlyph = extern struct {
     },
     glyph_col_1: [19]u8,
     glyph_col_2: [19]u8,
-    _pad: [3]u8,
+    _pad: [3]u8 = [_]u8{0} ** 3,
 };
 
 pub const HIIStringPackage = extern struct {

lib/std/os/uefi/protocols/simple_network_protocol.zig

@@ -126,9 +126,7 @@ pub const SimpleNetworkReceiveFilter = packed struct {
     receive_broadcast: bool,
     receive_promiscuous: bool,
     receive_promiscuous_multicast: bool,
-    _pad1: u3 = undefined,
-    _pad2: u8 = undefined,
-    _pad3: u16 = undefined,
+    _pad: u27 = 0,
 };
 
 pub const SimpleNetworkState = enum(u32) {
@@ -171,7 +169,5 @@ pub const SimpleNetworkInterruptStatus = packed struct {
     transmit_interrupt: bool,
     command_interrupt: bool,
     software_interrupt: bool,
-    _pad1: u4,
-    _pad2: u8,
-    _pad3: u16,
+    _pad: u28 = 0,
 };

lib/std/os/uefi/protocols/simple_text_input_ex_protocol.zig

@@ -64,14 +64,14 @@ pub const KeyState = extern struct {
         left_logo_pressed: bool,
         menu_key_pressed: bool,
         sys_req_pressed: bool,
-        _pad1: u21,
+        _pad: u21 = 0,
         shift_state_valid: bool,
     },
     key_toggle_state: packed struct {
         scroll_lock_active: bool,
         num_lock_active: bool,
         caps_lock_active: bool,
-        _pad1: u3,
+        _pad: u3 = 0,
         key_state_exposed: bool,
         toggle_state_valid: bool,
     },

lib/std/os/uefi/tables/boot_services.zig

@@ -239,9 +239,7 @@ pub const OpenProtocolAttributes = packed struct {
     by_child_controller: bool = false,
     by_driver: bool = false,
     exclusive: bool = false,
-    _pad1: u2 = undefined,
-    _pad2: u8 = undefined,
-    _pad3: u16 = undefined,
+    _pad: u26 = 0,
 };
 
 pub const ProtocolInformationEntry = extern struct {