[BUG] much of the gpg module does not work on Windows

Description

The gpg module does not generally work on Windows.

Setup

(Please provide relevant configs and/or SLS files (be sure to remove sensitive info).

Steps to Reproduce the behavior

Drop a gpg public key in c:\tmp, then...

• Try to import a key

[root@master1 salt]# salt win2012 gpg.import_key filename='c:\tmp\pubkey.gpg' user=salt
win2012:
    ERROR executing 'gpg.import_key': User SYSTEM does not exist
ERROR: Minions returned with non-zero exit code

[root@master1 salt]# salt win2012 gpg.import_key filename='c:\tmp\pubkey.gpg'
win2012:
    ERROR executing 'gpg.import_key': User SYSTEM does not exist
ERROR: Minions returned with non-zero exit code

• After manually importing the key, try to trust it:

[root@master1 salt]# salt win2012 gpg.trust_key keyid=A4E89A0D3328D483 trust_level=ultimately user=salt
win2012:
    ERROR: No mapping between account names and security IDs was done.
ERROR: Minions returned with non-zero exit code

[root@master1 salt]# salt win2012 gpg.trust_key keyid=A4E89A0D3328D483 trust_level=ultimately
win2012:
    ERROR executing 'gpg.trust_key': User SYSTEM does not exist
ERROR: Minions returned with non-zero exit code

Expected behavior

There is no documentation suggesting Windows is not supported, so these should work.

Versions Report

# salt win2012 test.versions_report
win2012:
    Salt Version:
               Salt: 3000.1
     
    Dependency Versions:
               cffi: 1.12.2
           cherrypy: 17.4.1
           dateutil: 2.8.0
          docker-py: Not Installed
              gitdb: 2.0.5
          gitpython: Not Installed
             Jinja2: 2.10.1
            libgit2: Not Installed
           M2Crypto: Not Installed
               Mako: 1.0.7
       msgpack-pure: Not Installed
     msgpack-python: 0.5.6
       mysql-python: Not Installed
          pycparser: 2.19
           pycrypto: Not Installed
       pycryptodome: 3.8.1
             pygit2: Not Installed
             Python: 3.5.4 (v3.5.4:3f56838, Aug  8 2017, 02:17:05) [MSC v.1900 64 bit (AMD64)]
       python-gnupg: 0.4.4
             PyYAML: 5.1.2
              PyZMQ: 18.0.1
              smmap: 2.0.5
            timelib: 0.2.4
            Tornado: 4.5.3
                ZMQ: 4.3.1
     
    System Versions:
               dist:   
             locale: cp1252
            machine: AMD64
            release: 2012ServerR2
             system: Windows
            version: 2012ServerR2 6.3.9600 SP0 Multiprocessor Free

Start with the gpg module documentation linked in the issue and reproduce gpg.import_key and gpg.trust_key on Windows using the shown commands. Investigate the reported user and account-mapping errors; done means the affected operations work on Windows or their unsupported behavior is clearly documented.