Add sysutils/sd-mux-ctrl, Tizen SD mux control program

The Tizen SD Mux is a SD card multiplexer: it electrically moves an SD
card between an SD reader and a device under test.

Details available at https://wiki.tizen.org/SD_MUX and
https://git.tizen.org/cgit/tools/testlab/sd-mux/

Thanks to Daniel Engberg and sbz for advice and updates.

Reviewed by: sbz
Approved by: sbz
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D29474