Added LogPrintf() calls to indicate state of fCheckpointsEnabled/the -checkpoints option.

Summary: Added LogPrintf() statements to indicate whether -checkpoints is set to on or off.

Test Plan:
ibd.sh -checkpoints=0 should output a line to debug.log stating checkpoints will not be verified.
ibd.sh -checkpoints=1 should output a line to debug.log stating checkpoints will be verified.

Reviewers: jasonbcox, Fabien, deadalnix, O1 Bitcoin ABC, #bitcoin_abc

Reviewed By: deadalnix, O1 Bitcoin ABC, #bitcoin_abc

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2638