libifconfig: Overhaul ifconfig_media_* interfaces

Define an ifmedia_t type to use for ifmedia words.

Add ifconfig_media_lookup_* functions to lookup ifmedia words by name.

Get media options as an array of option names rather than formatting it
as a comma-delimited list into a buffer.

Sprinkle const on static the static description tables for peace of
mind.

Don't need to zero memory allocated by calloc.

Reviewed by: kp
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D29029

libifconfig: Fix typo in symbol map

MFC after: 1 week

libifconfig: Add a function to get down reason

For use in ifconfig.

Reviewed by: kp
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D28991

libifconfig: Add missing symbols to map

Regenerate the list of generated symbols for libifconfig:

```
grep -hr ^ifconfig_sfp_ /usr/obj/usr/src/amd64.amd64/lib/libifconfig \
| sed 's/(.*/;/' | sort -u
```

Spotted by build failures caused by a missing symbol while working on
upgrading libifconfig from internal to private.

Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D28927

libifconfig: Add a symbol map

This is a no-op for now since libifconfig is only built as a static lib.

Reviewed by: freqlabs, kp, adrian
Differential Revision: https://reviews.freebsd.org/D28119