systemd service enhancements + run as system user

[RubenKelevra]

enable restart on crash and run as system user 'ipfs'

add some optional parameters with short help texts

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[Stebalien]

This unit is a minimal and conservative starting point, let's avoid adding too much.

[RubenKelevra]

This unit is a minimal and conservative starting point, let's avoid adding too much.

Ready for a new review :)

[Stebalien]

Thanks!