Fix 8-color PuTTY -nw sessions broken by a recent commit

Fix 8-color PuTTY -nw sessions broken by a recent commit

* src/term.c (init_tty): Fix the change which introduced support
for the terminfo Tc flag.  The code as installed broke colors in
PuTTY -nw sessions, because 'tigetflag' returned -1, which is
non-zero, so it was treated as the sign that true color is
supported.  But if the value returned by 'tigetflag' is -1, it means
the capability is not a boolean one, so that's not to be used as a
valid support for 24-bit color.  (Bug#44950)