ipc: linux: support incremental allowed ips updates

ipc: linux: support incremental allowed ips updates

Extend the interface of `wg set` to leverage the WGALLOWEDIP_F_REMOVE_ME
flag, a direct way of removing a single allowed ip from a peer,
allowing for incremental updates to a peer's configuration. By default,
allowed-ips fully replaces a peer's allowed ips using
WGPEER_REPLACE_ALLOWEDIPS under the hood. When '+' or '-' is prepended
to any ip in the list, wg clears WGPEER_F_REPLACE_ALLOWEDIPS and sets
the WGALLOWEDIP_F_REMOVE_ME flag on any ip prefixed with '-'.

$ wg set wg0 peer <PUBKEY> allowed-ips +192.168.88.0/24,-192.168.0.1/32

This command means "add 192.168.88.0/24 to this peer's allowed ips if
not present, and remove 192.168.0.1/32 if present".

Use -isystem so that headers in uapi/ take precedence over system
headers; otherwise, the build will fail on systems running kernels
without the WGALLOWEDIP_F_REMOVE_ME flag.

Note that this patch is meant to be merged alongside the kernel patch
that introduces the flag.

Signed-off-by: Jordan Rife <jordan@jrife.io>
[Jason: removed linux ifdefs; this will eventually come to other platforms too. minor style nits.]
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>