Add support for tcsh shell

[Rageoholic]

Problem

On FreeBSD, rustup will generate a .cargo/env that is sourced in .profile. There are two issues here.

1. tcsh does not source .profile on run, it sources .login
2. Manually sourcing .cargo/env will give a syntax error that too many arguments were given to case

Steps

1. Install FreeBSD on a fresh machine with tcsh as the set shell (I used a VM)
2. Install rustup using the given directions on the website
3. You can either try to source .cargo/env or exit and reenter the shell
4. Try running any rustup commands

Possible Solution(s)

• Modify .cargo/env to not use BASH specific extensions (I wouldn't quite know what specific syntax you should be using)
• Add sourcing .cargo/env to .login if present,

Notes

No response

Rustup version

Rustup 1.26.0

Installed toolchains

rustc 1.71.0

[rbtcollins]

rustup scatters a bunch of entries to source the env file, because we don't know what shell the user will be using (it may not be the one they installed with). We have some code that represents this and I don't think we have tcsh support - if you'd like to add that that would be great.

[rami3l]

AFAIK tcsh isn't even POSIX, so supporting that falls into the same category as supporting fish, meaning that:

• The source suggestion is incorrect anyway (even worse than rustup suggest executing "source ...", but this command is absent in dash #3415) because the env file cannot be read by tcsh, which is more or less expected.
• We need to add special cases one by one for each non-POSIX shell in the ideal case scenario, similar to what is done in Add support for fish shell #3108.