UHD 4.10 added two new stream/device arguments:

  - 'mtu'        (stream-arg)  -- override the link-detected MTU
  - 'force_mtu'  (device-arg)  -- skip MTU detection entirely

Both already flow through SoapyUHD without code change: setupStream()
passes its kwargs verbatim into stream_args_t::args via kwargsToDict(),
and make_uhd() does the same for device_args. They work today on UHD
4.10 -- they just aren't surfaced in getStreamArgsInfo().

Pothos GUI / SoapySDRUtil --probe / Sonneman / etc. enumerate the
ArgInfo list when populating UI knobs and hint dropdowns. Without an
ArgInfo entry, users have to know the magic key by reading UHD docs
and type it manually. Adding one ArgInfo struct surfaces 'mtu' in
every Soapy front-end automatically.

Use case: remote streaming over WAN where path-MTU discovery fails
(common with X410/X310 over consumer routers), or local network
topologies that interfere with auto-detection.

Guard with '#if UHD_VERSION >= 4100000' so the option only shows on
UHD versions that actually honor it. The matching 'force_mtu' device
arg already flows through; documenting it requires no ArgInfo (we
don't expose getDeviceArgsInfo at the Soapy layer).

No code change for the runtime path -- the kwargs already pipe to
stream_args_t::args. This commit is pure metadata advertisement.

Signed-off-by: Tom Hensel <code@jitter.eu>