Importprivkey shows address
Scratching an itch: make `importprivkey` output the corresponding address.
Without this PR, `importprivkey` shows no output. Because we're moving towards an "address-based" RPC interface, rather than "account-based", there's a gap when using `importprivkey` because there's no way to assign it to a specific account, but also no easy way to determine the address. This change fixes that wart.
Fixes for gcc 7
This fixes a few issues when using a newer compiler (in my case gcc 7.1.1) available in for example Arch Linux.
Solves for example this issue: https://github.com/zcash/zcash/issues/2304
The first thing is an error when checking for boost_system and is solved by disabling that warning (wich is treated as an error and, hence, stops the build):
```
configure:22242: checking for exit in -lboost_system-mt
configure:22267: g++ -m64 -o conftest -std=c++11 -pipe -fPIC -O1 -fwrapv -fno-strict-aliasing -Werror -g -Wformat -Wformat-security -Wstack-protector -fstack-protector-all -fPIE ..<snip>... /x86_64-unknown-linux-gnu/share/../lib conftest.cpp -lboost_system-mt -lanl >&5
conftest.cpp:70:6: error: declaration of 'char exit()' conflicts with built-in declaration 'void exit(int)' [-Werror=builtin-declaration-mismatch]
char exit ();
^~~~
cc1plus: all warnings being treated as errors
```
The second thing was to clean some code that is deprecated in C++11 which also lead to a warning treated as error. It could also be fixed with `-Wno-deprecated` but better to fix the issue in my opinion.
