Check against MANDATORY flags prior to accepting to mempool

Previously transactions were only tested again the
STANDARD_SCRIPT_VERIFY_FLAGS prior to mempool acceptance, so any bugs in
those flags that allowed actually-invalid transactions to pass would
result in allowing invalid transactions into the mempool. Fortunately
there is a second check in CreateNewBlock() that would prevent those
transactions from being mined, resulting in an invalid block, however
this could still be exploited as a DoS attack.
Add libbitcoinconsensus.pc to .gitignore
remove max orphan blocks config parameter since it is no longer functional
Merge pull request #5253

7c041b3 Check against MANDATORY flags prior to accepting to mempool (Peter Todd)
Merge pull request #5523

4bc95c4 remove max orphan blocks config parameter since it is no longer functional (Jameson Lopp)
Merge pull request #5518

1e22d45 Add libbitcoinconsensus.pc to .gitignore (Michael Ford)
Merge remote-tracking branch 'bitcoin/master' into develop