Simplify is_my_tty()

Simplify is_my_tty()

This commit will serve to document why we shouldn't worry about the
truncation in the call to strlcpy(3).  Since we have one more byte in
tmptty than in full_tty, truncation will produce a string that is at
least one byte longer than full_tty.  Such a string could never compare
equal, so we're actually handling the truncation in a clever way.  Maybe
too clever, but that's why I'm documenting it here.

Now, about the simplification itself:

Since we made sure that both full_tty and tmptty are null-terminated, we
can call strcmp(3) instead of strncmp(3).  We can also simplify the
return logic avoiding one branch.

Cc: Paul Eggert <eggert@cs.ucla.edu>
Signed-off-by: Alejandro Colomar <alx@kernel.org>
Reviewed-by: Iker Pedrosa <ipedrosa@redhat.com>