Fix inconsistent output of prefix and bugs in 'show' command

Fix inconsistent output of prefix and bugs in 'show' command

cmd_show_list function implements the 'show' command.

cmd_show_list output is inconsistent: it sometimes shows a prefix
and sometimes does not.
For example, in the below, you see that there is a prefix before
each value, except for 'enabled'.

    (gdb) show style
    style address background:  The "address" style background color is: none
    style address foreground:  The "address" style foreground color is: blue
    style address intensity:  The "address" style display intensity is: normal
    enabled:  CLI output styling is enabled.
    style filename background:  The "filename" style background color is: none
    ...

There are other inconsistencies or bugs e.g. in
the below we see twice insn-number-max, once with a prefix
and once without prefix : last line, just before the value of
instruction-history-size which is itself without prefix.

    (gdb) show record
    record btrace bts buffer-size:  The record/replay bts buffer size is 65536.
    record btrace cpu:  btrace cpu is 'auto'.
    record btrace pt buffer-size:  The record/replay pt buffer size is 16384.
    record btrace replay-memory-access:  Replay memory access is read-only.
    record full insn-number-max:  Record/replay buffer limit is 200000.
    record full memory-query:  Whether query if PREC cannot record memory change of next instruction is off.
    record full stop-at-limit:  Whether record/replay stops when record/replay buffer becomes full is on.
    function-call-history-size:  Number of functions to print in "record function-call-history" is 10.
    insn-number-max:  instruction-history-size:  Number of instructions to print in "record instruction-history" is 10.
    (gdb)

Also, some values are output several times due to some aliases, so avoid outputting duplicated
values by skipping all aliases.

Now that the command structure has a correct 'back-pointer' from a command
to its prefix command, we can simplify cmd_show_list by removing its prefix argument
and at the same time fix the output inconsistencies and bugs.

gdb/ChangeLog

2020-05-15  Philippe Waroquiers  <philippe.waroquiers@skynet.be>

	* cli/cli-setshow.h (cmd_show_list): Remove prefix argument.
	* cli/cli-decode.c (do_show_prefix_cmd): Likewise.
	* command.h (cmd_show_list): Likewise.
	* dwarf2/index-cache.c (show_index_cache_command): Likewise.
	* cli/cli-setshow.c (cmd_show_list): Use the prefix to produce the output.  Skip aliases.

gdb/testsuite/ChangeLog

2020-05-15  Philippe Waroquiers  <philippe.waroquiers@skynet.be>

	* gdb.base/default.exp: Update output following fixes.