Loading rc in non-interactive login shell

[t0fik]

Not loading rc files in exec mode was causing problems with setting up environment starting X session.

Changes:

• Shell works now like non-interactive bash, -l cause loading environment. 'Fixes' How to running script with rc in command line? #3374
• xexec implements options from bash/zsh exec command

[gforsyth]

Hey @t0fik -- thanks for putting this in! Can you rebase/merge master in? There are some spurious CI failures that we can eliminate that way. Thanks!

[t0fik]

@gforsyth master merged.

[anki-code]

@t0fik thanks for your work! Is this PR could fix #3443 ? Or I'm not clearly understand (could you give some cases).

[AstraLuma]

@anki-code when xonsh starts, it can be:

• Interactive or non-interactive
• Login or non-login

The old behavior only loaded RC files for interactive shells. This PR additionally loads RC files in the non-interactive+login case.

[t0fik]

@anki-code @astronouth7303 Exactly. The behaviour of Bash is the same.
Ex.
xonsh -c 'some command' - does not load rc files
xonsh -l -c 'some command' - loads environment from rc

[anki-code]

xonsh -l -c 'some command' - loads environment from rc

@t0fik great! Thanks! Am I right that

xonsh -l -rc rcfile1 rcfile2 -c 'print(var_from_rc)'

will works also?

[t0fik]

@anki-code Yes, it will work that way.
I've only disabled setting rc to None when -l switch was used. Rest of the process of loading rc files is unchanged.

[scopatz]

Going to close/reopen to trigger CI

[scopatz]

Hi @t0fik - this seems to have some problems on Windows and needs black to be run on it

[t0fik]

@scopatz I fixed tests on Windows and ran black. Other problems seems to be not related with this PR.
I've also merged master.

[scopatz]

So I have restarted CI a few times, and I think this is related to the test failures. Can you take a closer look please?

[t0fik]

@scopatz My code is all green now.

[gforsyth]

Thanks for pushing on this @t0fik !! This looks great.

[gforsyth]

Also, I see this is your first contribution to xonsh. Thank you for the contribution and welcome!!