Fix leak of set/show verbose doc, avoid xfree of static string

Fix leak of set/show verbose doc, avoid xfree of static string

In the tests
  py-pp-registration/gdb.log
  default/gdb.log
  foll-fork/gdb.log
  setshow/gdb.log
  break-interp/gdb.log
Valgrind detects a leak of the doc strings for the set and show verbose cmd.

Here is the stacktrace of the leaked set doc:
==25548== 15 bytes in 1 blocks are definitely lost in loss record 101 of 3,120
==25548==    at 0x4C2BE6D: malloc (vg_replace_malloc.c:309)
==25548==    by 0x409C27: xmalloc (common-utils.c:44)
==25548==    by 0x778AF9: xstrdup (xstrdup.c:34)
==25548==    by 0x3F860F: add_setshow_cmd_full(char const*, command_class, var_types, void*, char const*, char const*, char const*, void (*)(char const*, int, cmd_list_element*), void (*)(ui_file*, int, cmd_list_element*, char const*), cmd_list_element**, cmd_list_element**, cmd_list_element**, cmd_list_element**) [clone .constprop.10] (cli-decode.c:495)
==25548==    by 0x3F8ADB: add_setshow_boolean_cmd(char const*, command_class, int*, char const*, char const*, char const*, void (*)(char const*, int, cmd_list_element*), void (*)(ui_file*, int, cmd_list_element*, char const*), cmd_list_element**, cmd_list_element**) (cli-decode.c:593)
==25548==    by 0x3F7442: _initialize_cli_cmds() (cli-cmds.c:1768)
==25548==    by 0x69EED3: initialize_all_files() (init.c:365)
==25548==    by 0x658A84: gdb_init(char*) (top.c:2163)
==25548==    by 0x5403E1: captured_main_1 (main.c:863)
==25548==    by 0x5403E1: captured_main (main.c:1167)
==25548==    by 0x5403E1: gdb_main(captured_main_args*) (main.c:1193)
==25548==    by 0x289CA7: main (gdb.c:32)

The leak is created by top.c set_verbose 'elaborate joke':
the doc string is changed according to the verbosity:
  (gdb) help set verbose
  Set verbosity.
  (gdb) set verbose on
  (gdb) help set verbose
  Set verbose printing of informational messages.
  (gdb)

set_verbose creates the leak as it replaces the string allocated in
the above stacktrace by a static (non translated) string:
...
  if (info_verbose)
    {
      c->doc = "Set verbose printing of informational messages.";
...

Also, this can possibly trigger a call to 'free' of a static string,
as c->doc_allocated is kept true, while the string is not allocated anymore.

This patch:
 * fixes the leak by freeing the previous docs if doc_allocated.
 * internationalize the messages.
 * properly sets doc_allocated to 0 once doc strings are static.

gdb/ChangeLog
2018-12-28  Philippe Waroquiers  <philippe.waroquiers@skynet.be>

	* top.c (set_verbose): Free previous docs if doc_allocated.
	Internationalize messages.  Set doc_allocated to 0.