env: add spack env activate/deactivate and shell support

- `spack env activate foo`: sets SPACK_ENV to the current active env name

- `spack env deactivate`: unsets SPACK_ENV, deactivates the environment

- added support to setup_env.sh and setup_env.csh

- other env commands work properly with SPACK_ENV, as with an environment
  arguments.

- command-line --env arguments take precedence over the active
  environment, if given.

share/spack/csh/spack.csh

@@ -63,6 +63,47 @@ case cd:
         cd `\spack location $_sp_arg $_sp_args`
     endif
     breaksw
+case env:
+    shift _sp_args  # get rid of 'env'
+
+    set _sp_arg=""
+    [ $#_sp_args -gt 0 ] && set _sp_arg = ($_sp_args[1])
+
+    if ( "$_sp_arg" == "-h" ) then
+        \spack env -h
+    else
+        switch ($_sp_arg)
+            case activate:
+                set _sp_env_arg=""
+                [ $#_sp_args -gt 1 ] && set _sp_env_arg = ($_sp_args[2])
+
+                if ( "$_sp_env_arg" == "" || "$_sp_env_arg" =~ "-*" ) then
+                    # no args or does not start with -: just execute
+                    \spack $_sp_flags env $_sp_args
+                else
+                    shift _sp_args  # consume 'activate' or 'deactivate'
+                    # actual call to activate: source the output
+                    eval `\spack $_sp_flags env activate --csh $_sp_args`
+                endif
+                breaksw
+            case deactivate:
+                set _sp_env_arg=""
+                [ $#_sp_args -gt 1 ] && set _sp_env_arg = ($_sp_args[2])
+
+                if ( "$_sp_env_arg" != "" ) then
+                    # with args: execute the command
+                    \spack $_sp_flags env $_sp_args
+                else
+                    # no args: source the output
+                    eval `\spack $_sp_flags env deactivate --csh`
+                endif
+                breaksw
+            default:
+                echo default
+                \spack $_sp_flags env $_sp_args
+                breaksw
+        endsw
+    endif
 case use:
 case unuse:
 case load:
@@ -113,3 +154,4 @@ endsw
 _sp_end:
 unset _sp_args _sp_full_spec _sp_modtype _sp_module_args
 unset _sp_sh_cmd _sp_spec _sp_subcommand _sp_flags
+unset _sp_arg _sp_env_arg