Add --devel=sbs_category: append aircraft category as SBS field 24

[chrislanzara]

Summary

Adds a new --devel=sbs_category runtime flag that appends the aircraft
category as field 24 of the SBS output.

• Aircraft category (A0–D7) is decoded directly from ADS-B DF17
  identification messages (metype 1–4) — no external database or lookup
• Field 24 is emitted as a two-hex-char value (e.g. A3) on
  identification messages; empty on all other message types
• Field 23 (RSSI placeholder) is always emitted when sbs_category is
  active, populated only if --devel=sbs_rssi is also set — keeps
  field 24 at a fixed position regardless of whether RSSI is enabled
• Buffer allocation raised from 250 to 260 bytes when active
• Follows the same --devel= pattern established by sbs_rssi

Usage

--devel=sbs_category

Combined with RSSI:

--devel=sbs_rssi --devel=sbs_category

Note: comma-separated form (--devel=sbs_rssi,sbs_category) does not
work — the --devel= parser treats the comma as a name/value separator.
Use separate flags.

Example output (MSG,1 identification frames only)

MSG,1,...,EXS059B ,,,,,,,,,,,0,-29.5,A3
MSG,1,...,SRD226E ,,,,,,,,,,,0,-14.6,A1
MSG,1,...,RYR91XZ ,,,,,,,,,,,0,-6.3,A3

Field 24 is empty on MSG types 3–8.

[wiedehopf]

please use one flag for all your extra fields.
call it whatever you like.

--devel=sbs_extra_fields

removing the rssi devel option is ok, devel options have no stability guarantee.

[wiedehopf]

let's use a new PR so no force pushing is required.