option,main: Partial support of XDG base directory specification

- Add functions that are able to query XDG.
- Replace defaults for
  - &runtimepath. Does not follow #78.
  - &viewdir.
  - &undodir.
  - &directory.
  - &backupdir. Does not follow #78.
  - vimrc location.
- Remove user vimrc file line from :version message.

src/nvim/os/stdpaths.c

@@ -0,0 +1,100 @@
+#include "nvim/os/os.h"
+#include "nvim/strings.h"
+#include "nvim/path.h"
+#include "nvim/garray.h"
+
+typedef enum {
+  kXDGConfigHome,
+  kXDGDataHome,
+  kXDGCacheHome,
+  kXDGRuntimeDir,
+  kXDGConfigDirs,
+  kXDGDataDirs,
+} XDGDirType;
+
+static const char *xdg_env_vars[] = {
+  [kXDGConfigHome] = "XDG_CONFIG_HOME",
+  [kXDGDataHome] = "XDG_DATA_HOME",
+  [kXDGCacheHome] = "XDG_CACHE_HOME",
+  [kXDGRuntimeDir] = "XDG_RUNTIME_DIR",
+  [kXDGConfigDirs] = "XDG_CONFIG_DIRS",
+  [kXDGDataDirs] = "XDG_DATA_DIRS",
+};
+
+static const char *const xdg_defaults[] = {
+  // Windows, Apple stuff are just shims right now
+#ifdef WIN32
+  // Windows
+#elif APPLE
+  // Apple (this includes iOS, which we might need to handle differently)
+  [kXDGConfigHome] = "~/Library/Preferences",
+  [kXDGDataHome] = "~/Library/Application Support",
+  [kXDGCacheHome] = "~/Library/Caches",
+  [kXDGRuntimeDir] = "~/Library/Application Support",
+  [kXDGConfigDirs] = "/Library/Application Support",
+  [kXDGDataDirs] = "/Library/Application Support",
+#else
+  // Linux, BSD, CYGWIN
+  [kXDGConfigHome] = "~/.config",
+  [kXDGDataHome] = "~/.local/share",
+  [kXDGCacheHome] = "~/.cache",
+  [kXDGRuntimeDir] = "",
+  [kXDGConfigDirs] = "/etc/xdg/",
+  [kXDGDataDirs] = "/usr/local/share/:/usr/share/",
+};
+#endif
+
+static const char *get_xdg(XDGDirType idx)
+{
+  const char *env = xdg_env_vars[idx];
+  const char *fallback = xdg_defaults[idx];
+
+  const char *ret = os_getenv(env);
+  if (!ret && fallback) {
+    ret = (const char *)expand_env_save((char_u *)fallback);
+  }
+
+  return ret;
+}
+
+static const char *get_xdg_home(XDGDirType idx)
+{
+  const char *dir = get_xdg(idx);
+  if (dir) {
+    dir = (const char *)concat_fnames(dir, "nvim", true);
+  }
+  return dir;
+}
+
+static void create_dir(const char *dir, int mode, const char *suffix)
+{
+  char *failed;
+  if (!os_mkdir_recurse(dir, mode, &failed)) {
+    // TODO: Create a folder in $TMPDIR instead
+    DLOG("Create dir failed");
+  }
+}
+
+const char *get_user_conf_dir(void)
+{
+  return get_xdg_home(kXDGConfigHome);
+}
+
+const char *get_from_user_conf(const char * fname)
+{
+  return (const char *)concat_fnames(get_user_conf_dir(), fname, true);
+}
+
+const char *get_user_data_dir(void)
+{
+  return get_xdg_home(kXDGDataHome);
+}
+
+const char *get_from_user_data(const char * fname)
+{
+  const char *dir = (const char *)concat_fnames(get_user_data_dir(), fname, true);
+  if (!os_isdir((char_u *)dir)) {
+    create_dir(dir, 0755, fname);
+  }
+  return dir;
+}