[MEDIUM] http: make safer use of the DONT_READ and AUTO_CLOSE flags

[MEDIUM] http: make safer use of the DONT_READ and AUTO_CLOSE flags

Several HTTP analysers used to set those flags to values that
were useful but without considering the possibility that they
were not called again to clean what they did. First, replace
direct flag manipulation with more explicit macros. Second,
enforce a rule stating that any buffer which changes one of
these flags from the default must restore it after completion,
so that other analysers see correct flags.

With both this fix and the previous one about analyser bits,
we should not see any more stuck sessions.