nix-env -i: Add a flag ‘--remove-all’ / ‘-r’

This is equivalent to running ‘nix-env -e '*'’ first, except that it
happens in a single transaction.  Thus, ‘nix-env -i pkgs...’ replaces
the profile with the specified set of packages.

The main motivation is to support declarative package management
(similar to environment.systemPackages in NixOS).  That is, if you
have a specification ‘profile.nix’ like this:

  with import <nixpkgs> {};
  [ thunderbird
    geeqie
    ...
  ]

then after any change to ‘profile.nix’, you can run:

  $ nix-env -f profile.nix -ir

to update the profile to match the specification.  (Without the ‘-r’
flag, if you remove a package from ‘profile.nix’, it won't be removed
from the actual profile.)

Suggested by @zefhemel.

doc/manual/nix-env.xml

@@ -209,6 +209,10 @@ also <xref linkend="sec-common-options" />.</phrase></para>
     <arg choice='plain'><option>--preserve-installed</option></arg>
     <arg choice='plain'><option>-P</option></arg>
   </group>
+  <group choice='opt'>
+    <arg choice='plain'><option>--remove-all</option></arg>
+    <arg choice='plain'><option>-r</option></arg>
+  </group>
   <arg choice='plain' rep='repeat'><replaceable>args</replaceable></arg>
 </cmdsynopsis>
 
@@ -318,6 +322,16 @@ number of possible ways:
 
   </varlistentry>
 
+  <varlistentry><term><option>--remove-all</option></term>
+    <term><option>-r</option></term>
+
+    <listitem><para>Remove all previously installed packages first.
+    This is equivalent to running <literal>nix-env -e '*'</literal>
+    first, except that everything happens in a single
+    transaction.</para></listitem>
+
+  </varlistentry>
+
 </variablelist>
 
 </refsection>