Move various SigningProviders to signingprovider.{cpp,h}

Moves all of the various SigningProviders out of sign.{cpp,h} and
keystore.{cpp,h}. As such, keystore.{cpp,h} is also removed.

Includes and the Makefile are updated to reflect this. Includes were largely
changed using:
git grep -l "keystore.h" | xargs sed -i -e 's;keystore.h;script/signingprovider.h;g'

src/script/standard.cpp

@@ -9,7 +9,6 @@
 #include <pubkey.h>
 #include <script/script.h>
 
-
 typedef std::vector<unsigned char> valtype;
 
 bool fAcceptDatacarrier = DEFAULT_ACCEPT_DATACARRIER;