add systemd support

[Stebalien]

This change:

1. adds socket activation support for the API and the gateway
2. notifies systemd when go-ipfs is ready
3. notifies systemd when go-ipfs is shutting down

Unlike my previous attempt (#5514), this change merges the sockets passed by systemd with those configured in ipfs config. This is likely what the user wants.

Motivation:

1. Change 1 makes it possible to start go-ipfs on demand.
2. Change 2 makes it possible to wait for go-ipfs to finish starting. This makes it possible to use systemd's automount feature along with add experimental daemon plugin for 9P2000.L support #6612 to auto-start IPFS and auto-mount /ipfs on demand.

[hsanjuan]

Would you consider putting the sockets part as a separate, re-usable module out of the ipfs codebase? Seems quite useful.

[Stebalien]

@hsanjuan done.

[gmasgras]

tested this and it works as expected

[olizilla]

@Stebalien could you merge these systemd improvements into the stabalize-dht branch so we can try them on a gateway node pls?

[Stebalien]

@olizilla I've rebased the stabilize branch. However, that branch now requires 2048 bit keys so we'll want to wait until we've updated the bootstrappers (or add a patch to that branch to revert it).