Upgrade generated parse_line() to accept constant SBuf

[rousskov]

These changes attempt to preserve existing squid.conf grammar idiosyncrasies. A lot of squid.conf parsing code needs to be upgraded; these changes focus on parse_line() because upcoming reconfiguration improvements require that particular function to accept SBuf.

Also fixed ConfigParser SetCfgLine() method and CfgLine data member descriptions. These members are not about the whole squid.conf line, and not every squid.conf line goes through these members: These members are dedicated to tokens located after a directive name on a directive line. A lot of related code needs to be polished, but we fixed these docs because this change affects all SetCfgLine() callers and CfgLine users.