SourceLayout: add libconfiguration for squid.conf parser

[yadij]

This library is intended to present the API(s) for configuration processing, management and access to the rest of Squid.

Initial class Configuration::File combines:

a) file management design from ConfigParser::CfgFile,

Extended to load files in large chunks (up to 1MB) and parse lines in memory instead of via fgets().

b) '#line NN "filename" ' line changer from parseOneConfigFile()

c) FIFO pipe alternative input from parseOneConfigFile()

d) prefix whitespace tolerance from ConfigParser::strtokFile()

[yadij]

FTR: initial performance tests show this PR increases -k parse time by 4-6%

[yadij]

@rousskov, please check the remaining unresolved conversations for relevance.