rename jtag_nsrst_assert_width as adapter_nsrst_assert_width

rename jtag_nsrst_assert_width as adapter_nsrst_assert_width

Globally rename "jtag_nsrst_assert_width" as "adapter_nsrst_assert_width",
and move it out of the "jtag" command group ...  it needs to be used with
non-JTAG transports

Includes a migration aid (in jtag/startup.tcl) so that old user scripts
won't break.  That aid should Sunset in about a year.

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