We'll need to be able to work with debug adapter interfaces (drivers)
even when they're not used for JTAG ... for example, while there are
multi-transport drivers which support JTAG *and* several other
transports (or just one more, like SWD) there are also adapters
with more limited goals (and no JTAG support at all).

Start decoupling the two concepts ("debug adapter driver", "jtag")
by having two command groups, which initialize separately.

This will help us support OpenOCD sessions using only non-JTAG
transports, in which JTAG commands should not be registered.
Update docs to mention that the JTAG, SVF, and XSVF commands
won't work without a JTAG transport.

Note that at least commands working with SRST are still inappropriately
coupled  to JTAG ... inappropriate because (a) SRST is not part of the
JTAG standard, for all that many platforms (like ARM) expect it; and also
(b) because they're used with non-JTAG debug and programming interfaces,
too.  They should perhaps become generic "interface" operations at some
point.  (Similarly with the clock rate to be used by a given adapter.)

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>