core/execute: add the magic character '!' to allow privileged execution (#3493)

This patch implements the new magic character '!'. By putting '!' in front
of a command, systemd executes it with full privileges ignoring paramters
such as User, Group, SupplementaryGroups, CapabilityBoundingSet,
AmbientCapabilities, SecureBits, SystemCallFilter, SELinuxContext,
AppArmorProfile, SmackProcessLabel, and RestrictAddressFamilies.

Fixes partially https://github.com/systemd/systemd/issues/3414
Related to https://github.com/coreos/rkt/issues/2482

Testing:
1. Create a user 'bob'
2. Create the unit file /etc/systemd/system/exec-perm.service
   (You can use the example below)
3. sudo systemctl start ext-perm.service
4. Verify that the commands starting with '!' were not executed as bob,
   4.1 Looking to the output of ls -l /tmp/exec-perm
   4.2 Each file contains the result of the id command.

`````````````````````````````````````````````````````````````````
[Unit]
Description=ext-perm

[Service]
Type=oneshot
TimeoutStartSec=0
User=bob
ExecStartPre=!/usr/bin/sh -c "/usr/bin/rm /tmp/exec-perm*" ;
    /usr/bin/sh -c "/usr/bin/id > /tmp/exec-perm-start-pre"
ExecStart=/usr/bin/sh -c "/usr/bin/id > /tmp/exec-perm-start" ;
    !/usr/bin/sh -c "/usr/bin/id > /tmp/exec-perm-star-2"
ExecStartPost=/usr/bin/sh -c "/usr/bin/id > /tmp/exec-perm-start-post"
ExecReload=/usr/bin/sh -c "/usr/bin/id > /tmp/exec-perm-reload"
ExecStop=!/usr/bin/sh -c "/usr/bin/id > /tmp/exec-perm-stop"
ExecStopPost=/usr/bin/sh -c "/usr/bin/id > /tmp/exec-perm-stop-post"

[Install]
WantedBy=multi-user.target]
`````````````````````````````````````````````````````````````````

1 files changed, 4 insertions, 3 deletions

diff --git a/man/systemd.service.xml b/man/systemd.service.xml
index 6641dfed4f..6e969abc25 100644
--- a/man/systemd.service.xml
+++ b/man/systemd.service.xml
@@ -295,9 +295,10 @@
         If the absolute filename is prefixed with
         <literal>-</literal>, an exit code of the command normally
         considered a failure (i.e. non-zero exit status or abnormal
-        exit due to signal) is ignored and considered success. If both
-        <literal>-</literal> and <literal>@</literal> are used, they
-        can appear in either order.</para>
+        exit due to signal) is ignored and considered success.
+        If the absolute path is prefixed with <literal>!</literal> then
+        it is executed with full privileges. <literal>-</literal>, <literal>@</literal>, and <literal>!</literal>
+        may be used together and they can appear in any order.</para>
 
         <para>If more than one command is specified, the commands are
         invoked sequentially in the order they appear in the unit