jtag/tcl.c cleanup -- split out "adapter.c"

Clean up the jtag/tcl.c file, which was one of the biggest and
messiest ones in that directory.  Do it by splitting out all the
generic adapter commands to a separate "adapter.c" file (leaving
the "tcl.c" file holding only JTAG utilities).

Also rename the little-used "jtag interface" to "adapter_name", which
should have been at least re-categorized earlier (it's not jtag-only).

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

doc/openocd.texi

@@ -2093,12 +2093,12 @@ target.
 @end deffn
 
 @deffn Command {interface_list}
-List the interface drivers that have been built into
+List the debug adapter drivers that have been built into
 the running copy of OpenOCD.
 @end deffn
 
-@deffn Command {jtag interface}
-Returns the name of the interface driver being used.
+@deffn Command {adapter_name}
+Returns the name of the debug adapter driver being used.
 @end deffn
 
 @section Interface Drivers