mrsh does not set PS1 prompt

Although it displays `$` (or `#` when root), it never actually sets the `PS1` prompt to this.

Relevant code: https://github.com/emersion/mrsh/blob/60ef497e4d83320be5c32505b244f0f0318e4879/shell/entry.c#L60

The other POSIX shells I tested, `bash --posix` and `dash`, both set the PS1, even if it is as simple as `$` in `dash`'s case.

Start in shell/entry.c around line 60, where mrsh displays the default prompt, and compare the behavior with bash --posix and dash. Done means PS1 is set to the displayed "$" or "#" prompt according to the shell's privilege context.

Written by the indexing model from the issue text.