bindings: Better error handling in enumeration get() function.

Prior to this patch a call to get() with an invalid enumeration value
would raise an exception that would, for example, terminate the python
interpreter, whereas now the exception is handled and translated into a
proper python exception.
colead-slm: Fix serial port timeout.
ols: Fix serial port timeout.
build: Require glib 2.34.
Fix sr_dev_has_option().

This wasn't taking the SR_CONF_GET/_SET/_LIST flags into account.

Thanks to Janne Huttunen for spotting this.
usb: Simplify usb_get_port_path() and fix writing into a const char *.
README: Bump glib version to 2.34.
Add two new units.

Add degrees and henrys to the list of supported units. Degree is
an unit of plane angle where one degree is 1/360th of a full circle.
Henry is the SI unit of inductance.
Add measured quantities for LCR meters.

Add parallel and serial model inductance, capacitance and resistance
for LCR meter measurements. Add also secondary quantities many LCR
meters calculate, namely dissipation factor, quality factor and phase
angle.
Add quantity and flag for difference measurements.

Add 'SR_MQ_DIFFERENCE' quantity for reporting relative difference
between the current measurement and the reference value. The value
of this quantity will normally be reported in percents. Add also
the flag 'SR_MQFLAG_REFERENCE' for indicating that the reported
value is not the current measurement but the reference value instead.
Add two new 'auto' flags.

Add flags for indicating that the meter has selected the measured
quantitiy and/or the used measurement model automatically. These
are similar to the existing auto-range flag.
Add config key for LCR meter type.

Add a config key that can be used to indicate that the device is
an LCR meter.
Add config key for output frequency.

Add a config key for getting/setting the output frequency. The value
will be an uint64 and measured in hertz.
Add pretty-printer for new units and flags.

Add support for printing henries and degrees as well as the new
reference and auto flags.
Add protocol decoder for Cyrustek ES51919 LCR meter chip.

Add a protocol decoder for the Cyrustek ES51919 LCR meter chip.
This chipset (together with ES51920 front-end) is supposedly used
by multiple different portable LCR meters including at least
DER EE DE-5000, Yihua V&A VA520, Mastech MS5308, Uni-T UT612,
CEM DT-9935 and various OEM rebadges of them.

The communication protocol seems to be implemented on the Cyrustek
chip itself so all the different models are expected to use the
same protocol if they implement a host connection. Unfortunately
the protocol is not available in the public documentation of the
chipset, so this implementation is based on reverse engineering it
from traffic captures.

The actual connection between the meter and the host computer may be
different from meter to meter even when based on the same chip. This
module implements a decoder for the protocol and some common helper
functions for interfacing with the meter via an RS-232 serial port.
Add driver for DER EE DE-5000 LCR meter.

Add a driver for the DER EE DE-5000 LCR meter. This meter is based on
the Cyrustek ES51919/ES51920 chipset and communicates with the host
computer via an optional connectivity kit.

The kit uses an optoisolated unidirectional link to connect to the
meter and an USB cable on the host side. Internally the connection is
using the FTDI FT232R USB UART chip i.e. from the host computer point
of view the meter is connected into an RS-232 serial port.

This driver implements just a thin shim layer for registering the
driver and uses the es51919 module for all the actual work.
libsigrok-internal.h: Fix some outdated comments.
Rename SR_CONF_LCR_METER to SR_CONF_LCRMETER.

(to be consistent with some other key names)
scpi-pps: Split boolean set options into enable/disable.

This makes it easier to support devices that need something else
than "ON" or "OFF".
configure.ac: Drop ols Windows exception, should be obsolete.

(see #205)