Make it possible to opt-out of the centralized alert system by providing
an option `-noalerts` or `-alerts=0`. The default remains unchanged.
This is a gentler form of #6260, in which I went a bit overboard by
removing the alert system completely.
I intend to add this to the GUI options in another pull after this.
Github-Pull: #6274
Rebased-From: 02a6702a82a5b00e0e0351041dd3267308b7f319
strUsage+=HelpMessageOpt("-?",_("This help message"));
strUsage+=HelpMessageOpt("-alerts",strprintf(_("Receive and display P2P network alerts (default: %u)"),DEFAULT_ALERTS));
strUsage+=HelpMessageOpt("-alertnotify=<cmd>",_("Execute command when a relevant alert is received or we see a really long fork (%s in cmd is replaced by message)"));
strUsage+=HelpMessageOpt("-blocknotify=<cmd>",_("Execute command when the best block changes (%s in cmd is replaced by block hash)"));
strUsage+=HelpMessageOpt("-checkblocks=<n>",strprintf(_("How many blocks to check at startup (default: %u, 0 = all)"),288));