Move libsnark in-repo as a git subtree
This PR pulls in the libsnark subtree at the exact commit that we currently fetch via the depends system. To verify:
```
$ ./contrib/devtools/git-subtree-check.sh src/snark
src/snark in HEAD was last updated to upstream commit 9ada3f84ab484c57b2247c2f41091fd6a0916573 (tree c10a38c759)
src/snark in HEAD currently refers to tree 34e916d3f6
:100644 100644 427f4f4ce913e54da68b M Makefile
:040000 040000 42f29e42d1dd73536163 M src
FAIL: subtree directory tree doesn't match subtree commit tree
```
This shows that there are changes relative to what we currently use, due to the later commits in the PR. If we exclude them, we see that the code is identical:
```
$ git checkout 26a8f68ea8
$ ./contrib/devtools/git-subtree-check.sh src/snark
src/snark in HEAD was last updated to upstream commit 9ada3f84ab484c57b2247c2f41091fd6a0916573 (tree c10a38c759)
src/snark in HEAD currently refers to tree c10a38c759
GOOD
```
Closes#820.
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.
Previously various user-facing strings have used inconsistent currency units "BTC",
"btc" and "bitcoins". This adds a single constant and uses it for each reference to
the currency unit.
Also adds a description of the unit for --maxtxfee, and adds the missing "amount"
field description to the (deprecated) move RPC command.
Corrections are to the median block times, which were generated by subtracting
CBlockIndex::nMedianTimeSpan / 2 from the block height and then multiplying by
the target spacing. GetMedianTimePast() takes an array sorted by std::sort() and
returns element CBlockIndex::nMedianTimeSpan / 2, meaning that if
CBlockIndex::nMedianTimeSpan is odd (which it is), there is an out-by-one error
in the subtraction.
