All PDs: Drop unneeded exceptions.

In all current PDs it is not necessary to raise an exception upon
invalid states (of the PD's state machine), since we can guarantee that
no such invalid state can ever be reached in these PDs.
Various PDs: Throw SamplerateError instead of Exception.

Also, use the "if not self.samplerate" form, which catches both the case
where self.samplerate is None, as well as the case where it is 0.
Various PDs: Minor consistency fixes.

(Consistently use single-quotes for all Python strings everywhere)
uart/i2cfilter: Don't check multiple-choice options.

For options which only have a limited set of valid values, we don't need
to check (in the PD) whether a valid value was supplied, since the backend
can do that for us.
All PDs: Minor whitespace and consistency fixes.

 - No newlines at the end of files.
 - No trailing ';' characters.
 - Comparison with None: Use 'is None' or 'is not None'.
 - Comparison with True/False: Use 'if cond:' or 'if not cond:'.
 - Various minor whitespace fixes.
Fix incorrect doxygen comment for srd_decoder_list().

This fixes bug #378.
runtc: Update to the new libsigrok session API.

(also fixes the build for libsigrokdecode)
Drop references to obsolete sigrok-commits mailing list.
tests/check_session.c: Fix compiler warning.

This fixes 'warning: "_POSIX_C_SOURCE" redefined' with some Python
versions.
tests/pdtest: Small fix to make it work with Python 3.2.

Python 3.2 doesn't yet have the copy() method for lists:

  AttributeError: 'list' object has no attribute 'copy'
nrf24l01: prettify annotation output