Lay groundwork for Windows support

Move Unix-specific code to util-unix.cpp, and place Windows equivalents
in util-win32.cpp.  Most of the Windows functions are just stubs at
the moment, and we need a build system that works on Windows.

util-win32.cpp

@@ -0,0 +1,124 @@
+/*
+ * Copyright 2014 Andrew Ayer
+ *
+ * This file is part of git-crypt.
+ *
+ * git-crypt is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * git-crypt is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with git-crypt.  If not, see <http://www.gnu.org/licenses/>.
+ *
+ * Additional permission under GNU GPL version 3 section 7:
+ *
+ * If you modify the Program, or any covered work, by linking or
+ * combining it with the OpenSSL project's OpenSSL library (or a
+ * modified version of that library), containing parts covered by the
+ * terms of the OpenSSL or SSLeay licenses, the licensors of the Program
+ * grant you additional permission to convey the resulting work.
+ * Corresponding Source for a non-source form of such a combination
+ * shall include the source code for the parts of OpenSSL used as well
+ * as that of the covered work.
+ */
+
+#include <io.h>
+#include <stdio.h>
+#include <fcntl.h>
+#include <windows.h>
+
+std::string System_error::message () const
+{
+	std::string	mesg(action);
+	if (!target.empty()) {
+		mesg += ": ";
+		mesg += target;
+	}
+	if (error) {
+		// TODO: use FormatMessage()
+	}
+	return mesg;
+}
+
+void	temp_fstream::open (std::ios_base::openmode mode)
+{
+	close();
+
+	char			tmpdir[MAX_PATH + 1];
+
+	DWORD			ret = GetTempPath(sizeof(tmpdir), tmpdir);
+	if (ret == 0) {
+		throw System_error("GetTempPath", "", GetLastError());
+	} else if (ret > sizeof(tmpdir) - 1) {
+		throw System_error("GetTempPath", "", ERROR_BUFFER_OVERFLOW);
+	}
+
+	char			tmpfilename[MAX_PATH + 1];
+	if (GetTempFileName(tmpdir, TEXT("git-crypt"), 0, tmpfilename) == 0) {
+		throw System_error("GetTempFileName", "", GetLastError());
+	}
+
+	filename = tmpfilename;
+
+	std::fstream::open(filename.c_str(), mode);
+	if (!std::fstream::is_open()) {
+		DeleteFile(filename.c_str());
+		throw System_error("std::fstream::open", filename, 0);
+	}
+}
+
+void	temp_fstream::close ()
+{
+	if (std::fstream::is_open()) {
+		std::fstream::close();
+		DeleteFile(filename.c_str());
+	}
+}
+
+void	mkdir_parent (const std::string& path)
+{
+	std::string::size_type		slash(path.find('/', 1));
+	while (slash != std::string::npos) {
+		std::string		prefix(path.substr(0, slash));
+		if (GetFileAttributes(prefix.c_str()) == INVALID_FILE_ATTRIBUTES) {
+			// prefix does not exist, so try to create it
+			if (!CreateDirectory(prefix.c_str(), NULL)) {
+				throw System_error("CreateDirectory", prefix, GetLastError());
+			}
+		}
+
+		slash = path.find('/', slash + 1);
+	}
+}
+
+std::string our_exe_path () // TODO
+{
+	return argv0;
+}
+
+int exec_command (const char* command, std::ostream& output) // TODO
+{
+	return -1;
+}
+
+int exec_command_with_input (const char* command, const char* p, size_t len) // TODO
+{
+	return -1;
+}
+
+bool successful_exit (int status) // TODO
+{
+	return status == 0;
+}
+
+static void	init_std_streams_platform ()
+{
+	_setmode(_fileno(stdin), _O_BINARY);
+	_setmode(_fileno(stdout), _O_BINARY);
+}