Try to increase file descriptor rlimit if necessary

As the default can be too low, especially on OSX.

src/util.h

@@ -197,6 +197,7 @@ bool WildcardMatch(const std::string& str, const std::string& mask);
 void FileCommit(FILE *fileout);
 int GetFilesize(FILE* file);
 bool TruncateFile(FILE *file, unsigned int length);
+int RaiseFileDescriptorLimit(int nMinFD);
 void AllocateFileRange(FILE *file, unsigned int offset, unsigned int length);
 bool RenameOver(boost::filesystem::path src, boost::filesystem::path dest);
 boost::filesystem::path GetDefaultDataDir();