Expose temporary flag for key()

Expose the temporary flag for the key() command so that keys can expire 
as soon as the bitlash call ends.  This allows the use of temporary keys
without introducing a key leak that could eventually fill the key table
with no way to free them.  For example:

loop()
{
  hq.report("uptime", key(uptime.minutes));
  delay(60000);
}

would fill the key table in ~13 minutes (there's 51 default keys,
and a key table max size of 64).  With this update

loop()
{
  hq.report("uptime", key(uptime.minutes, 1));
  delay(60000);
}

would run forever since the key is freed as soon as the hq.report command
ends.

Two notes: The comments in key.c indicate there was thought of having the keys time-out after a certain interval
indicated by the "at" variable, but that hasn't been implemented yet.  This code may need to be tweak if/when that happens.

Secondly, these changes will also need to be made to keyMap_int() if/when that pull-request is approved.