Move the event mask into a platform-neutral product

`InotifyEventMask` took its bits from the C header, so nothing that
imported it could build outside Linux. The new `InotifyMask` product
spells out the kernel constants instead; a Linux test compares each
of them with the header. `Inotify` re-exports the module, so
existing code is unaffected.

Sources/InotifyMask/InotifyEventMask.swift

@@ -0,0 +1,51 @@
+/// The events and flags of an inotify watch or event, as bits.
+///
+/// The values are the constants of the Linux `<sys/inotify.h>` header,
+/// which are part of the kernel's stable interface. Spelling them out here
+/// keeps this module free of the C header, so it builds on every platform
+/// and lets code that only stores or compares masks be tested off Linux.
+public struct InotifyEventMask: OptionSet, Sendable, Hashable {
+	public let rawValue: UInt32
+
+	public init(rawValue: UInt32) {
+		self.rawValue = rawValue
+	}
+
+	// MARK: - Watchable Events
+
+	public static let access = InotifyEventMask(rawValue: 0x0000_0001)
+	public static let modify = InotifyEventMask(rawValue: 0x0000_0002)
+	public static let attrib = InotifyEventMask(rawValue: 0x0000_0004)
+	public static let closeWrite = InotifyEventMask(rawValue: 0x0000_0008)
+	public static let closeNoWrite = InotifyEventMask(rawValue: 0x0000_0010)
+	public static let open = InotifyEventMask(rawValue: 0x0000_0020)
+	public static let movedFrom = InotifyEventMask(rawValue: 0x0000_0040)
+	public static let movedTo = InotifyEventMask(rawValue: 0x0000_0080)
+	public static let create = InotifyEventMask(rawValue: 0x0000_0100)
+	public static let delete = InotifyEventMask(rawValue: 0x0000_0200)
+	public static let deleteSelf = InotifyEventMask(rawValue: 0x0000_0400)
+	public static let moveSelf = InotifyEventMask(rawValue: 0x0000_0800)
+
+	// MARK: - Combinations
+
+	public static let move: InotifyEventMask = [.movedFrom, .movedTo]
+	public static let close: InotifyEventMask = [.closeWrite, .closeNoWrite]
+	public static let allEvents: InotifyEventMask = [
+		.access, .attrib, .closeWrite, .closeNoWrite,
+		.create, .delete, .deleteSelf, .modify,
+		.moveSelf, .movedFrom, .movedTo, .open,
+	]
+
+	// MARK: - Watch Flags
+
+	public static let onlyDir = InotifyEventMask(rawValue: 0x0100_0000)
+	public static let dontFollow = InotifyEventMask(rawValue: 0x0200_0000)
+	public static let oneShot = InotifyEventMask(rawValue: 0x8000_0000)
+
+	// MARK: - Kernel-Only Flags
+
+	public static let unmount = InotifyEventMask(rawValue: 0x0000_2000)
+	public static let queueOverflow = InotifyEventMask(rawValue: 0x0000_4000)
+	public static let ignored = InotifyEventMask(rawValue: 0x0000_8000)
+	public static let isDir = InotifyEventMask(rawValue: 0x4000_0000)
+}