serial: Improved docs.
README.devices: Cosmetics, consistency fixes, updates.
Add initial support for the UNI-T UT60G.
Add support for the Tenma 72-7750 (UNI-T UT60G rebadge).
serial-dmm: Support the Tenma 72-7745 via UT-D02 cable.

(it was already supported in uni-t-dmm via UT-D04 cable)
hardware/common/dmm: Fix debug output level.

Most messages from the DMM parsers are not hard errors, lower to
sr_dbg() so that the sigrok-cli output doesn't get cluttered (by default)
with debug output such as:

 P1: 0.001100 V DC AUTO
 sr: fs9721: Sync nibble in byte 0 (0x00) is invalid.
 P1: 0.001100 V DC AUTO

(using -l 4 or -l 5 will still allow the user to see such messages)
serial-dmm: Drop unused DMM_COUNT.
Add sr_session_save_init().

This allows a frontend to initialize a session file, providing the
required samplerate and probe names, without having a proper
struct sr_dev_inst handy.

sr_session_append() is then used to add captured data to the session
file, as usual.

The existing sr_session_save() function works the same way as always.
Add SR_CONF_MAX_UNCOMPRESSED_SAMPLES.
Add support for SR_CONF_MAX_UNCOMPRESSED_SAMPLES.
ikalogic-scanalogic2: Publish SR_CONF_MAX_UNCOMPRESSED_SAMPLES.
zeroplus-logic-cube: Publish SR_CONF_MAX_UNCOMPRESSED_SAMPLES.
zeroplus-logic-cube: List all keys in device options.
ikalogic-scanalogic2: List all keys in device options.
chronovu-la8: Publish SR_CONF_MAX_UNCOMPRESSED_SAMPLES.
atten-pps3xxx: Initial driver skeleton.
Add config keys for programmable power supplies.
atten-pps3xxx: Full support for the PPS3203T-3S.
atten-pps3xxx: Use configured values to construct packet.

This used the values previously received from the device, not very useful.
atten-pps3xxx: Push configured settings even without acquisition.