libpcap: add rpcapd as package

libpcap: add rpcapd as package

This enables building of rpcapd and adds it as a package.

It is a daemon that allows remote packet capturing from another machine.
E.g. Wireshark can talk to it using the Remote Capture Protocol (RPCAP).
https://www.tcpdump.org/manpages/rpcapd.8.html

Compile and run tested:
OpenWrt 21.02.0-rc4 r16256-2d5ee43dc6 on x86/64 and mvebu/cortexa9

Signed-off-by: Stephan Schmidtmer <hurz@gmx.org>
(cherry picked from commit 891c8676a1602d31adf3ab9f913664ae0d3b4029)