Fixes #1497 ZCA-009 by restricting data exporting to user defined folder.

Previously the RPC interface allowed z_exportwallet, backupwallet and
dumpwallet to write data to an arbitrary filename.  ZCA-009 demonstrates
how this is vulnerable.  The resolution is to only allow data to
written when the -exportdir has been configured.  Also filenames are
restricted to alphanumeric characters.

src/utilstrencodings.h

@@ -22,6 +22,7 @@
 /** This is needed because the foreach macro can't get over the comma in pair<t1, t2> */
 #define PAIRTYPE(t1, t2)    std::pair<t1, t2>
 
+std::string SanitizeFilename(const std::string& str);
 std::string SanitizeString(const std::string& str);
 std::vector<unsigned char> ParseHex(const char* psz);
 std::vector<unsigned char> ParseHex(const std::string& str);