add -x parameter for aria2c in apt-fast.conf (forgot in 95f79b1)
add --min-split-size=1M parameter for aria2c to enhance parallel

the default value is 20M, too large for normal deb packages
add -s parameter for aria2c to enhance parallel

the default falue is 5, is user set MAXNUM larger than 5, it will be limited by it
Merge pull request #64 from PeterDaveHello/enhance_aria2c_parallel

Enhance aria2c parallel