My recent fix for programmable function keys in syscons only worked

My recent fix for programmable function keys in syscons only worked
when TEKEN_CONS25 is configured.  Fix this by adding a function to
set the flag that enables the fix and always calling this function
for syscons.

Expand the man page for teken_set_cons25().  This function is not
very useful since it can only set but not clear 1 flag.  In practice,
it is only used when TEKEN_CONS25 is configured and all that does is
choose the the default emulation for syscons at compile time.