avocado/__main__.py: rework module entry point

The following things were changed and for the following reasons:

1) Remove the current hack to force the program name at the module
entry point, for obvious reasons.

2) Return the exit status code, for obvious reasons.

3) Add some safety with a `__main__` check, because Sphinx will
load the avocado module in such a way that the avocado app is run
and the build fails. This is true on my system, and may be the case
on others.
Signed-off-by: NCleber Rosa <crosa@redhat.com>

avocado/__main__.py

@@ -6,8 +6,6 @@ import sys
 
 from avocado.cli.app import AvocadoApp
 
-if sys.argv[0].endswith('__main__.py'):
-    sys.argv[0] = 'python -m avocado'
-
-main = AvocadoApp()
-main.run()
+if __name__ == '__main__':
+    main = AvocadoApp()
+    sys.exit(main.run())

avocado/cli/parser.py

@@ -22,6 +22,9 @@ import argparse
 
 from avocado.version import VERSION
 
+PROG = 'avocado'
+DESCRIPTION = 'Avocado Test Runner'
+
 
 class Parser(object):
 
@@ -31,9 +34,9 @@ class Parser(object):
 
     def __init__(self):
         self.application = argparse.ArgumentParser(
-            prog='avocado',
+            prog=PROG,
             add_help=False,  # see parent parsing
-            description='Avocado Test Runner')
+            description=DESCRIPTION)
         self.application.add_argument('-v', '--version', action='version',
                                       version='Avocado %s' % VERSION)
         self.application.add_argument('--plugins', action='store',
@@ -49,8 +52,10 @@ class Parser(object):
         """
         self.args, _ = self.application.parse_known_args()
 
-        # Use parent parsing to avoid to break the output of --help option
-        self.application = argparse.ArgumentParser(parents=[self.application])
+        # Use parent parsing to avoid breaking the output of --help option
+        self.application = argparse.ArgumentParser(prog=PROG,
+                                                   description=DESCRIPTION,
+                                                   parents=[self.application])
 
         # Subparsers where Avocado subcommands are plugged
         self.subcommands = self.application.add_subparsers(