(u)mount: broken on BSD (incl. MacOS)

[diktomat]

Current Behavior

1. Completing mount and umount uses lsblk and /proc/mounts to find devices. Both ways are Linux only and fail on BSDs, including MacOS as BSD-derived OS.
2. Completing mount and umount presents completions for LABEL=, ID= etc, which are also Linux only.

Expected Behavior

Another way to find mounted devices (maybe just plain mount?) and devices to mount (diskutil list -plist?) is used, wrong completions are removed.

Steps To Reproduce

1. On BSD/Darwin
2. Type umount <tab>
3. The following will be displayed (annotated):

~ > umount <tab>
# all local directories, regardless of them being (parent to) a mount point or not
Applications/
[...]
# errors
ERR  (lsblk resolves to executable in current directory (./lsblk))
ERR1                (open /proc/mounts: no such file or directory)
# wrong completions
ID=                         (specifies device by udev hardware ID)
LABEL=                      (specifies device by filesystem label)
PARTLABEL=                   (specifies device by partition label)
PARTUUID=                     (specifies device by partition UUID)
UUID=                        (specifies device by filesystem UUID)

Version

carapace-bin 1.5.4

OS

• Darwin
• Linux
• Termux
• Windows

Shell

• Bash
• Elvish
• Fish
• Nushell
• Oil
• Powershell
• Xonsh
• Zsh

Anything else?

No response

[rsteube]

OS-specific completers/actions and variants aren't implemented yet and thus a lot is still very linux focussed.
Its' simple to to do a GOOS check in the action, but the whole thing needs a fitting concept for the long run.

related #1326
related #2356