kconfig: nconf: refactor in print_in_middle()

kconfig: nconf: refactor in print_in_middle()

This helper is the same as the sample code in the NCURSES HOWTO [1],
but it is over-engineering to be used for nconf.

I do not see any good reason to use the 'float' type just for the
division by 2.

All the call-sites pass a non-NULL pointer to the first argument,
so 'if (win == NULL) win = stdscr;' is dead code.

'if (startx != 0) x = startx;' is dead code because 'x' will be
overridden some lines below, by 'x = startx + (int)temp;'.

All the call-sites pass a non-zero value to the second argument,
so 'if (starty != 0)' is always true.

getyx(win, y, x) is also dead-code because both 'y' and 'x' are
overridden.

All the call-sites pass 0 to the third parameter, so 'startx' can
be removed.

All the call-sites pass a non-zero value to the fourth parameter,
so 'if (width == 0) width = 80;' is dead code.

The window will be refreshed later, so there is no need to call
refresh() in this function.

Change the type of the last parameter from 'chtype' to 'int' to be
aligned with the prototype, 'int wattrset(WINDOW *win, int attrs);'

I also slightly cleaned up the indentation style.

[1]: https://tldp.org/HOWTO/NCURSES-Programming-HOWTO/color.html

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>