ifconfig(8) IEEE802.11 SSID format

Currently `ifconfig wlanX scan` will show SSID in hex when the scanned SSID
is non-printable. And the non-printable character is when the character
with value < 32 or value >= 126 in ifconfig(8)'s implementation. That leads
to a problem that other languages using UTF-8 or other encoding will be
treated as non-printable in ifconfig(8), thus not printed on screen.

Here is the possible way to solve the problem:
When a SSID is going to be printed, copy the byte stream (uint8_t *) into
wide character stream (wchar_t *), and use libc iswprint(3) to check
whether characters are printable or non-printable. I have tested this and
it is workable when the locale is correctly set.