dmesg: print only 2 hex digits for each hex-escaped byte

dmesg: print only 2 hex digits for each hex-escaped byte

As buf is passed as a signed char buffer in fwrite_hex, fprintf will
print every byte from 0x80 as a signed-extended int causing each of
these bytes to be printed as "\xffffff80" and such, which can be pretty
confusing. Force fprintf to use the argument as a char to make it print
only 2 digits, e.g. "\x80".

Signed-off-by: Ivan Delalande <colona@arista.com>