gpython: Fix how --long-option=value=something is processed

Carlos reports that when pymain is invoked as

    > python --login='uid=username,ou=people,dc=something,dc=de' my_script.py

it crashes as

    Traceback (most recent call last):
      File "/srv/slapgrid/slappart5/software_release/bin/python", line 312, in <module>
        pymain(sys.argv)
      File "/opt/slapgrid/133edb8b6bfc135bce30900e2b50555e/parts/pygolang/gpython/__init__.py", line 113, in pymain
        for (opt, arg) in igetopt:
      File "/opt/slapgrid/133edb8b6bfc135bce30900e2b50555e/parts/pygolang/gpython/__init__.py", line 552, in __next__
        opt, arg = opt.split('=')

While @jerome correctly notices that the problem here is due to --login
is passed to python instead of my_script.py it still highlights a
problem on gpython side in its _IGetOpt parser for which I made a thinko
in 26058b5b (gpython: Factor-out options parsing into getopt-style
_IGetOpt helper) without considering that a value for
--long-option=value could itself contain another '=' symbols.

-> Fix this thinko.

Without the fix gpython --unknown=x=y crashes as

    Traceback (most recent call last):
      File "/home/kirr/src/wendelin/venv/py39.venv/bin/gpython", line 8, in <module>
        sys.exit(main())
      File "/home/kirr/src/tools/go/pygolang-master/gpython/__init__.py", line 402, in main
        for (opt, arg) in igetopt:
      File "/home/kirr/src/tools/go/pygolang-master/gpython/__init__.py", line 562, in __next__
        opt, arg = opt.split('=')
    ValueError: too many values to unpack (expected 2)

but after the fix it reports more user-friendly

    RuntimeError: unexpected option --unknown

/reported-and-reviewed-by @vnmabus
/reported-on https://lab.nexedi.com/nexedi/pygolang/-/issues/1
/reviewed-on !32

gpython/__init__.py

 #!/usr/bin/env python
 # -*- coding: utf-8 -*-
-# Copyright (C) 2018-2024  Nexedi SA and Contributors.
+# Copyright (C) 2018-2025  Nexedi SA and Contributors.
 #                          Kirill Smelkov <kirr@nexedi.com>
 #
 # This program is free software: you can Use, Study, Modify and Redistribute
@@ -559,7 +559,7 @@ class _IGetOpt:
         # long option
         arg = None
         if '=' in opt:
-            opt, arg = opt.split('=')
+            opt, arg = opt.split('=', 1)
         if opt not in self._opts:
             raise RuntimeError('unexpected option %s' % opt)
         arg_required = self._opts[opt]

gpython/gpython_test.py

 # -*- coding: utf-8 -*-
-# Copyright (C) 2019-2024  Nexedi SA and Contributors.
+# Copyright (C) 2019-2025  Nexedi SA and Contributors.
 #                          Kirill Smelkov <kirr@nexedi.com>
 #
 # This program is free software: you can Use, Study, Modify and Redistribute
@@ -404,6 +404,23 @@ def test_pymain_ver(runtime):
     ret, out, err = _pyrun(['--version'], stdout=PIPE, stderr=PIPE, env=gpyenv(runtime))
     assert (ret, out, b(err)) == (0, b'', b(vok))
 
+# pymain --unknown/-Z option
+# gpython_only because output differs from !gpython
+@gpython_only
+def test_pymain_unknown():
+    from golang import b
+    def check(argv, errok):
+        ret, out, err = _pyrun(argv, stdout=PIPE, stderr=PIPE)
+        assert b(errok) in b(err)
+        assert ret != 0
+
+    check(['-Z'],                   "unexpected option -Z")
+    check(['-Z=xyz'],               "unexpected option -Z")
+    check(['-Z=xyz=pqr'],           "unexpected option -Z")
+    check(['--unknown'],            "unexpected option --unknown")
+    check(['--unknown=xyz'],        "unexpected option --unknown")
+    check(['--unknown=xyz=pqr'],    "unexpected option --unknown")
+
 # verify that ./bin/gpython runs ok.
 @gpython_only
 def test_pymain_run_via_relpath():