KRandom: Don't use up 16K of /dev/urandom to seed rand().

According to a bug report KRandom reads 16K from /dev/urandom when
seeding its RNG (which is just the libc RNG). This is because we simply
use QFile, which buffers by default, the fix is to avoid buffering the
output of this special file.

BUG:359485
REVIEW:127094
FIXED-IN:5.20.0