sync_client and other command line tools shouldn't use "fatal" for incorrect command line options

[brong]

#3 0x0000000000407fb3 in main (argc=6, argv=0x7ffc41da3808) at imap/sync_client.c:1009
1009 fatal("No replication mode specified", EX_USAGE);

This causes core dumps in the Fastmail environment, which is annoying!

[elliefm]

Fixed for sync_client by 70600b5

I had a look for other command places doing this (ack 'fatal.*EX_USAGE' and just ack EX_USAGE) and the rest all seem like legit fatal calls from service processes, or at least from things that have not been discovered until after work has started (e.g. cyr_virusscan validating search strings)