Shell: Exit the shell on (interactive) EOF with empty buffer

In other words, if the user presses EOF (normally Ctrl+D), we now print
out "<EOF>" and exit the shell without error.

Fixes SerenityOS#701.

Shell/LineEditor.cpp

@@ -302,6 +302,13 @@ String LineEditor::get_line(const String& prompt)
  }
  continue;
  }
+ if (ch == g.termios.c_cc[VEOF]) { // Normally ^D
+ if (m_buffer.is_empty()) {
+ printf("<EOF>\n");
+ exit(0);
+ }
+ continue;
+ }
  if (ch == 0x05) { // ^E
  if (m_cursor < m_buffer.size()) {
  printf("\033[%dC", m_buffer.size() - m_cursor);