Fix bit numbering

Bits seem to be numbered from 1 to N, which is very confusing since
almost every software starts at zero. Also it's common to find macros
like :

  #define BIT(N) (1 << (N))

and here that would introduce mistakes since one constantly has to
think about shifting bits before using the macro (eg: BIT(3) to check
bit called "4" in the spec, corresponding to value 0x8).

Since there are not that many places where this happens, this commit
aims at fixing them. Note that the hexadecimal values for each of them
were already correct and non-ambiguous.
Merge pull request #631 from wtarreau/fix-bit-numbering

Number bits starting from zero