Check to see if input is pending
int SLang_input_pending (int tsecs)
SLang_input_pending may be used to see if an input character
is available to be read without causing SLang_getkey to block.
It will wait up to tsecs tenths of a second if no characters
are immediately available for reading. If tsecs is less than
zero, then SLang_input_pending will wait -tsecs
milliseconds for input, otherwise tsecs represents 1/10
of a second intervals.
Not all systems support millisecond resolution.
SLang_getkey