uart: handle two stop bits configuration

Sample and process multiple STOP bits as specified, add 2.0 to the list
of supported configurations. This implementation works as expected with
integer numbers of STOP bits (0, 1, 2). For half-bits the sample point
as well as the annotation position will be incorrect (as a result of an
internal implementation detail of the existing decoder which is not easy
to address).

This commit reduces the diff size, and remains backwards bug-compatible.
Fixing the bit boundaries in annotations including support for half-bits
is more involved, and remains for a later commit.