cjtag: Add cJTAG OSCAN1 decoder.

[Note: This is a commit from Kongou Hikari ("diodep" on GitHub) that was
"rebased" by Uwe Hermann on top of the current libsigrokdecode mainline JTAG
decoder. There are various reasons for this, including avoiding non-mainline
or outdated decoder changes, as well as making it easily visible what the
changes vs. the current JTAG decoder are, in case we later need to apply some
changes to both decoders or in case both decoders might be merged later on.
Minor cosmetic and naming changes were also squashed in (no functional
changes, though).]