AK: Enable Core::File to switch blocking-mode during use

This closely mirrors Socket::set_blocking. Note that it does not make
sense to make this a virtual method on a base class, since
SeekableStream and FixedMemoryStream cannot possible be anything except
than blocking.

Userland/Libraries/LibCore/File.cpp

@@ -182,4 +182,13 @@ ErrorOr<void> File::truncate(size_t length)
     return System::ftruncate(m_fd, length);
 }
 
+ErrorOr<void> File::set_blocking(bool enabled)
+{
+    // NOTE: This works fine on Serenity, but some systems out there don't support changing the blocking state of certain POSIX objects (message queues, pipes, etc) after their creation.
+    // Therefore, this method shouldn't be used in Lagom.
+    // https://github.com/SerenityOS/serenity/pull/18965#discussion_r1207951840
+    int value = enabled ? 0 : 1;
+    return System::ioctl(fd(), FIONBIO, &value);
+}
+
 }

Userland/Libraries/LibCore/File.h

@@ -68,6 +68,13 @@ public:
     virtual ErrorOr<size_t> seek(i64 offset, SeekMode) override;
     virtual ErrorOr<void> truncate(size_t length) override;
 
+    // Sets the blocking mode of the file. If blocking mode is disabled, reads
+    // will fail with EAGAIN when there's no data available to read, and writes
+    // will fail with EAGAIN when the data cannot be written without blocking
+    // (due to the send buffer being full, for example).
+    // See also Socket::set_blocking.
+    ErrorOr<void> set_blocking(bool enabled);
+
     template<OneOf<::IPC::File, ::Core::MappedFile> VIP>
     int leak_fd(Badge<VIP>)
     {