nconfig: Fix segfault when help contains special characters

nconfig segfaults when help text contains the character '%'. For a quick
example, navigate to the kernel compression options and get the help for
bzip2. Doing so triggers a call to mvwprintw() with a string containing
'%' and no extra arguments to fill in the specifier's value. Fix this
case by printing the literal string retrieved from the kconfig.

 #0  0x00002b52b6b11d83 in vfprintf () from /lib/libc.so.6
 #1  0x00002b52b6bad010 in __vsnprintf_chk () from /lib/libc.so.6
 #2  0x00002b52b623991b in _nc_printf_string () from
 /lib/libncursesw.so.5
 #3  0x00002b52b6234cff in vwprintw () from /lib/libncursesw.so.5
 #4  0x00002b52b6234db9 in mvwprintw () from /lib/libncursesw.so.5
 #5  0x00000000004151d8 in fill_window (win=0x21b64c0,
     text=0x21b62b0 "CONFIG_KERNEL_BZIP2:\n\nIts compression ratio and
     speed is intermediate.\nDecompression speed is slowest among the
     three.  The kernel\nsize is about 10% smaller with bzip2, in
     compar...