input/vcd: rework VCD file import logic (vector, integer, real)

input/vcd: rework VCD file import logic (vector, integer, real)

Extend and rework the VCD input module: accept more data types, improve
usability, fix known issues.

Add support for bit vectors (arbitrary width), multi-bit integer values
(absolute 64bit width limit, internal limitation to single precision),
and floating point numbers ('real' in VCD, single precision in sigrok).
Unfortunately sigrok neither has concepts of multi-bit logic channels
nor IEEE-1364 stdlogic values, the input module maps input data to
strict boolean and multiple logic channels. A vector's channels are
named and grouped to reflect their relation. VCD 'integer' types are
mapped to sigrok analog channels. Add support for scoped signal names,
and the re-use of one VCD signal name for multiple variables.

Rework file and text handling. Only skip pointless UTF-8 BOMs before
file content (not between sections). Handle lack of line termination at
the end of the input file. Process individual lines of input chunks,
avoid glib calls when they'd result in malloc pressure, and severely
degrade performance. Avoid expensive string operations in hot loops.

Rearrange the order of parse steps, to simplify maintenance and review:
end of section, new section, timestamp, data values, unsupported. Flush
previously queued values in the absence of a final timestamp. Unbreak
$comment sections in the data part. Apply stricter checks to input data,
and propagate errors. Avoid silent operation (weak warnings can go
unnoticed) which yields results that are unexpected to users. Unbreak
the combination of 'downsample' with 'skip' and 'compress'. Reduce noise
when users limit the number of channels while the input file contains
more data (keep a list of consciously ignored channels). Do warn or
error out for serious and unexpected conditions.

Address minor issues. Use common support for datafeed submission. Keep
user specified options across file re-load. Fixup data type nits, move
complex code blocks into separate routines. Sort the groups of routines,
put helpers first and concentrate public routines at the bottom. Extend
the builtin help text. Update comments, update the copyright for the
non-trivial changes.

Fixes bug #776 by adding support for bit vectors.
Fixes bug #1476 by flushing most recently received sample data.