Add const qualifiers

Hackers,

I noticed that there was a mismatch between the const qualifiers for 
excludeDirContents in src/backend/backup/backup/basebackup.c and 
src/bin/pg_rewind/file_map.c and that led me to use ^static const.*\*.*= 
to do a quick search for similar cases.

I think at the least we should make excludeDirContents match, but the 
rest of the changes seem like a good idea as well.

Regards,
-David