PR: Check for `bsd` in sys.platform and use `sh` as a default shell on BSD systems

[spchamp]

On BSD platforms, the sys.platform test for a default shell in confpage.py may fall through to the Darwin case. This may result in an error as follows, also preventing the display of the Spyder Preferences window.

Traceback (most recent call last):
  File "/usr/home/gimbal/wk/python_wk/spyder_sandbox/env/lib/python3.11/site-packages/spyder/plugins/preferences/plugin.py", line 276, in <lambda>
    lambda: self.open_dialog(main.prefs_dialog_size))
            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/home/gimbal/wk/python_wk/spyder_sandbox/env/lib/python3.11/site-packages/spyder/plugins/preferences/plugin.py", line 256, in open_dialog
    container.create_dialog(
  File "/usr/home/gimbal/wk/python_wk/spyder_sandbox/env/lib/python3.11/site-packages/spyder/plugins/preferences/widgets/container.py", line 58, in create_dialog
    page.initialize()
  File "/usr/home/gimbal/wk/python_wk/spyder_sandbox/env/lib/python3.11/site-packages/spyder/plugins/preferences/api.py", line 86, in initialize
    self.setup_page()
  File "/usr/home/gimbal/wk/python_wk/spyder_sandbox/env/lib/python3.11/site-packages/spyder_terminal/confpage.py", line 55, in setup_page
    if mac_ver >= LooseVersion('10.15.0'):
       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/home/gimbal/wk/python_wk/spyder_sandbox/env/lib/python3.11/site-packages/setuptools/_distutils/version.py", line 90, in __ge__
    c = self._cmp(other)
        ^^^^^^^^^^^^^^^^
  File "/usr/home/gimbal/wk/python_wk/spyder_sandbox/env/lib/python3.11/site-packages/setuptools/_distutils/version.py", line 350, in _cmp
    if self.version == other.version:
       ^^^^^^^^^^^^
AttributeError: 'LooseVersion' object has no attribute 'version'

This patch adds a test for 'bsd' in sys.platform, here using 'sh' as a default shell for confpage.py

When the Spyder Issue reporter popped up with this issue, I'd originally reported it to the spyder project. Noticing the source references in the backtrace available in the Issue reporter window, I'll try to retract or close the issue there.

Fixes #342.

[spchamp]

tested on FreeBSD 13.2