Disable dumping regression for non-dumping builds (#3046)

Disable dumping regression for non-dumping builds (#3046)

`let_shadowing.smt2` uses dumping to test our printing infrastructure.
Since some builds do not support dumping, this commit disables that
regression for non-dumping builds. Additionally, it enables an error
message when trying to dump with a muzzled build and corrects the output
of `--show-config` to indicate that muzzled builds cannot dump.
Previously, the dumping output of a muzzled build was just silently
empty.

Most of the changes in `dump.cpp` are due to reformatting.