Changes

libosmo-trx/trxc: enlarge the params buffer

The SETFH command (implemented by trxcon and fake_trx) carries the
whole Mobile Allocation as pairs of Rx/Tx frequencies in kHz:

  CMD SETFH <HSN> <MAIO> <RXF1> <TXF1> [... <RXFN> <TXFN>]

With up to 64 ARFCNs in the Mobile Allocation, the parameters string
alone can exceed 1000 characters, far beyond the old 128 byte limit.

Derive OSMO_TRXC_PARAMS_LEN_MAX from OSMO_TRXC_MSG_BUF_SIZE, so that
any message ("RSP " + verb + status + params) still fits the
recommended socket buffer size.  Also add the OSMO_TRXC_CMD_SETFH
verb constant and a regression test doing a round-trip of a maximum
size SETFH command (64 ARFCNs).

Change-Id: I2f65b213f5ada499eea4abae87d3727057e03e22