* Add new command "INPUTCHAR" which checks for a pending key press and reports it to the FIFO (empty string if no key was pressed). This is better than INPUTTIMEOUT for implementing things like "Press ESC to skip this step" after all. Documented the command in the manpage. * libusplash.[hc]: Add flush_stdin() to clear stdin. Call it on usplash startup in main(), which fixes getting random key presses when calling usplash in a running system.