Support GEM_HOME, add limited support for user-installed gems (#1436)

The rehash process will now discover executables in additional locations:
- `~/.gem/ruby/<version>/bin/*`
- `$GEM_HOME/bin`

The `rbenv which` (and thus `rbenv exec`) command will also search these locations when looking up a command. This enables shims to dispatch calls to executables added by `gem install --user-install`.

Note that this support is limited:
- It will only work with C Ruby, as it's difficult to guess the `~/.gem/<engine>/<version>` directory for other Rubies without actually loading Ruby;
- It will only work for RBENV_VERSION values in the format `X.Y.Z` and not "system".

libexec/rbenv-rehash

@@ -104,6 +104,15 @@ list_executable_names() {
       echo "${file##*/}"
     done
   done
+  # list user-install'ed executables
+  for file in ~/.gem/ruby/*/bin/*; do
+    echo "${file##*/}"
+  done
+  if [ -n "$GEM_HOME" ]; then
+    for file in "$GEM_HOME"/bin/*; do
+      echo "${file##*/}"
+    done
+  fi
 }
 
 # The basename of each argument passed to `make_shims` will be