vty: clamp configured snaplen to the wire-framing limit
osmo-pcap carries each captured packet in a frame whose length field (struct osmo_pcap_data.len) is a uint16_t, so any snaplen above ~64 KiB cannot be transported and is silently clamped by the server when sizing its receive buffer (calc_data_max_len() caps at UINT16_MAX). The VTY, however, advertised libpcap's MAXIMUM_SNAPLEN (262144), misleading users into configuring values that never take effect.
Introduce OSMO_PCAP_MAX_SNAPLEN (65535) and, in both the client "pcap snaplen" and server "max-snaplen" handlers, warn and cap the value to it when a larger one is given. The command syntax keeps the <1-262144> range for backwards compatibility so existing configs still parse; the help text now documents the effective 64 KiB limit.
