script: shorten jtag_* command names

Drop the get_/set_/_pin/_list noise from the JTAG commands (e.g.
jtag_get_probes_list -> jtag_probes, jtag_set_spi_cs_pin -> jtag_spi_cs,
jtag_spi_rd_wr -> jtag_spi_xfer). jtag_open_probe -> jtag_open (not
jtag_probe, which would clash with jtag_probes under tab-completion).

Hard rename, no aliases. Updates the state-dump emitter, help text,
example script and docs accordingly.

scripts/example_script.txt

@@ -1,4 +1,4 @@
 
-jtag_open_probe 1
+jtag_open 0
 jtag_autoinit
-jtag_set_mode 0 EXTEST
+jtag_mode 0 EXTEST