split out options from read_config()

There is no longer any reason for these to be intertwined.

This deliberately avoids touching some files as much as possible because
they are super tangled and due to be replaced.  Those files are:

* fdroidserver/build.py
* fdroidserver/update.py

# Conflicts:
#	tests/testcommon.py

# Conflicts:
#	fdroidserver/btlog.py
#	fdroidserver/import_subcommand.py

tests/testcommon.py

@@ -21,6 +21,7 @@ import tempfile
 import unittest
 
 from pathlib import Path
+from unittest import mock
 
 
 class TmpCwd:
@@ -72,3 +73,16 @@ def mkdir_testfiles(localmodule, test):
     testdir = testroot / unittest.TestCase.id(test)
     testdir.mkdir(exist_ok=True)
     return tempfile.mkdtemp(dir=testdir)
+
+
+def parse_args_for_test(parser, args):
+    """Only send --flags to the ArgumentParser, not test classes, etc."""
+
+    from fdroidserver.common import parse_args
+
+    flags = []
+    for arg in args:
+        if arg[0] == '-':
+            flags.append(flags)
+    with mock.patch('sys.argv', flags):
+        parse_args(parser)