Fix function keys for syscons in cons25 mode (vidcontrol -T cons25).

kbd(4) (but only documented in atkbd(4)) maintains a table of strings
for 96 function keys. Using teken broke this 9+ years ago for the
most usable first 12 function keys and for 10 cursor keys, by supplying
its own non-programmable strings so that the keyboard driver's strings
are not used.

Fix this by supplying NULL in the teken layer for syscons in cons25 mode
so that the the strings are found in the kbd(4) layer.

vt needs more changes to use kbd(4)'s tables. Teken's cons25 table is
still needed to supply nonempty strings for vt in cons25 mode.

Keep using teken's xterm tables for both syscons and vt in xterm mode.
Function keys should at least default to xterm values in xterm mode,
and kbd(4) doesn't support this.

teken_set_cons25() sets a sticky flag to ask for the fix, and space is
reserved for another new flag. vt should set this flag when it uses
kbd(4)'s tables.

PR: 226553 (for vt)