Handle doas in place of sudo if available, outside of OpenBSD

[kit-ty-kate]

Alpine Linux officially started the migration from sudo to doas in Alpine 3.15 (https://wiki.alpinelinux.org/wiki/Release_Notes_for_Alpine_3.15.0#Move_from_sudo_to_doas).

It would be interesting to automatically use doas if it is present in place of sudo when sudo is not available (or if configured to do so). There doesn’t seem to be any automated way to detect if one or the other is configured so I’d say we should probably:

• If doas is available but sudo is not: use doas
• If sudo is available but doas is not: use sudo
• If both are available: ask the user which one they want to use and store it in the opam config

[dra27]

For the case where they're both found, let's "prefer" sudo (on the basis that if it and doas are present then sudo was added explicitly and probably configured) but then display a warning that sudo has been used and advise the required opam config command to set the preference permanently between doas and sudo (cf. various git things)