Added DSCP support
Refactored DSCP-related code a bit
Minor DSCP-related improvements

- Improved usage annotation
- Shifted DSCP value to set not whole DS field, but only it's DSCP part
Improved dscp parameter description
Merge branch 'dscp-support' of https://github.com/iavael/aria2 into iavael-dscp-support

Conflicts:
	src/prefs.cc
	src/prefs.h
	src/usage_text.h
Fix longer line in --dscp option help
Document --dscp option in manual page