`wglink`: control Wireguard interfaces

Everything related lives under /etc/wglink. All paths below are relative to that.
Each managed WG interface is configured by files under /etc/wglink/<interface>/.
Configuration for the node, the [Interface] file, is <interface>/self.conf.
Each peer has its own config file under <interface>/peers/*.conf. Files other than *.conf are ignored. Not having any peer files is not an error.
IP address(es) can be configured with <interface>/ipv4 and / or <interface>/ipv6; the contents should be a CIDR. If neither exists, no IP address is assigned.
Interfaces assumed point to point.
Need privileges to run wg, ip link, and ip address; uses grep.

Summary of command options

wglink list - shows names of all interfaces managed by wglink. Note: any WG interfaces not managed by wglink are not shown.
wglink create <interface> - create an IP interface, keep it down. The interface must not exist, but its configuration files must be in order.
wglink up <interface> - put the interface online; must be previously down.
wglink down <interface> - put the interface offline; must be previously up.
wglink destroy <inerface> - remove the interface; must be previously down.
wglink reload <interface> - reload peer files for the interface (wg syncconf). The interface may be either up or down.
wglink status {<interface>} - list status of given or all managed interface(s); outputs one of the following:
- "up" - the interface is up and running;
- "down" - the interface is configured but down;
- "absent" - the interface is not present (e.g. not created);
- "unknown" - the interface is not managed by wglink.
wglink help- display this message and exit.