support customizing install directory with $DPATH

This introduces the $DPATH environment variable to override the user's home directory just for D tools.

This is also intended to be used by DUB (dlang/dub#2281) when there is not the more explicit `$DUB_HOME` variable set on the system.

This allows users who install their D compiler through the install.sh script to both have the compiler installations as well as the dub packages be stored elsewhere in the system than in their home folder.

In case we have other tools that download their dependencies into the user's home directory, we should also make them support the same $DPATH environment variable.

script/install.sh

@@ -176,13 +176,21 @@ GET_PATH_AUTO_INSTALL=0
 GET_PATH_COMPILER=dc
 # Set a default install path depending on the POSIX/Windows environment.
 if posix_terminal; then
-    ROOT=~/dlang
+    if [ -z "$DPATH" ]; then
+        ROOT=~/dlang
+    else
+        ROOT="$DPATH/dlang"
+    fi
 else
-    # Default to a ROOT that is outside the POSIX-like environment.
-    if [ -z "$USERPROFILE" ]; then
-        fatal '%USERPROFILE% should not be empty on Windows.';
+    if [ -z "$DPATH" ]; then
+        # Default to a ROOT that is outside the POSIX-like environment.
+        if [ -z "$USERPROFILE" ]; then
+            fatal '%USERPROFILE% should not be empty on Windows.';
+        fi
+        ROOT=$(posix_path "$USERPROFILE")/dlang
+    else
+        ROOT=$(posix_path "$DPATH")/dlang
     fi
-    ROOT=$(posix_path "$USERPROFILE")/dlang
 fi
 TMP_ROOT=
 DUB_VERSION=