BUG/MINOR: log/balance: detect if user tries to use unsupported algo

BUG/MINOR: log/balance: detect if user tries to use unsupported algo

b61147fd ("MEDIUM: log/balance: merge tcp/http algo with log ones")
introduced some ambiguities, because while it shares some algos with the
ones from mode {tcp,http}, we forgot report an error when the user tries
to use an algorithm that is not available in this mode (as per the doc).

Because of that, haproxy would silently drop log messages during runtime.

To fix that, we ensure that algo is one of the supported ones during log
backend postparsing. If the algo is not supported, we raise an error.

This should be backported in 2.9 with b61147fd