Let's do the check early and skip most of the file if appropriate. Also, treat
missing $TERM same as "dumb". We're almost certainly at a dump terminal in that
case.
In https://github.com/systemd/systemd/issues/39114 users are reporting
that our script overrides PROMPT_COMMAND that they had. After looking
at /etc/bashrc in Fedora, I see that it only sets PROMPT_COMMAND if
[ -z "$PROMPT_COMMAND" ]. Let's adjust the script so this continues to
work.
Fixes https://github.com/systemd/systemd/issues/39114.
(This is a bit of a stretch. 39114 was originally about SecureCRT,
but that was resolved in SecureCRT. But there was a lot of dicussion
about the prompt being overriden, which this commit should fix.)
While we can safely assume that `/proc/sys/kernel/random/boot_id`
exists, the same can't be said for `/etc/machine-id` in environments
where systemd is installed, but not running. An example would be OCI
containers like with the official Arch Linux image, see [0].
Without this check the prompt would constantly output `/etc/machine-id:
no such file or directory` with the OSC events introduced in dadbb34
(v258).
[0] https://gitlab.archlinux.org/archlinux/archlinux-docker/-/issues/107
