GDB: Add NUMBER' completion to set' integer commands

Fix a completion consistency issue with `set' commands accepting integer
values and the special `unlimited' keyword:

(gdb) complete print -elements
print -elements NUMBER
print -elements unlimited
(gdb)

vs:

(gdb) complete set print elements
set print elements unlimited
(gdb)

(there is a space entered at the end of both commands, not shown here)
which also means if you strike <Tab> with `set print elements ' input,
it will, annoyingly, complete to `set print elements unlimited' right
away rather than showing a choice between `NUMBER' and `unlimited'.

Add `NUMBER' then as an available completion for such `set' commands:

(gdb) complete set print elements
set print elements NUMBER
set print elements unlimited
(gdb)

Adjust the testsuite accordingly.  Also document the feature in the
Completion section of the manual in addition to the Command Options
section already there.

gdb/NEWS

@@ -27,6 +27,18 @@
   emit to indicate where a breakpoint should be placed to break in a function
   past its prologue.
 
+* Completion now also offers "NUMBER" for "set" commands that accept
+  a numeric argument and the "unlimited" keyword.  For example:
+
+   (gdb) set width <TAB>
+   NUMBER     unlimited
+
+  and consequently:
+
+   (gdb) complete set width
+   set width NUMBER
+   set width unlimited
+
 * New commands
 
 maintenance set ignore-prologue-end-flag on|off

gdb/cli/cli-decode.c

@@ -989,6 +989,8 @@ integer_unlimited_completer (struct cmd_list_element *ignore,
       NULL,
     };
 
+  if (*text == '\0')
+    tracker.add_completion (make_unique_xstrdup ("NUMBER"));
   complete_on_enum (tracker, keywords, text, word);
 }
 

gdb/doc/gdb.texinfo

@@ -1960,6 +1960,21 @@ After displaying the available possibilities, @value{GDBN} copies your
 partial input (@samp{b make_} in the example) so you can finish the
 command.
 
+If the command you are trying to complete expects either a keyword or a
+number to follow, then @samp{NUMBER} will be shown among the available
+completions, for example:
+
+@smallexample
+(@value{GDBP}) print -elements @key{TAB}@key{TAB}
+NUMBER     unlimited
+(@value{GDBP}) print -elements@tie{}
+@end smallexample
+
+@noindent
+Here, the option expects a number (e.g., @code{100}), not literal
+@code{NUMBER}.  Such metasyntactical arguments are always presented in
+uppercase.
+
 If you just want to see the list of alternatives in the first place, you
 can press @kbd{M-?} rather than pressing @key{TAB} twice.  @kbd{M-?}
 means @kbd{@key{META} ?}.  You can type this either by holding down a

gdb/testsuite/gdb.base/settings.exp

@@ -215,8 +215,14 @@ proc test-integer {variant} {
 	test_gdb_complete_none \
 	    "$set_cmd "
     } else {
+	test_gdb_complete_multiple "$set_cmd " "" "" {
+	    "NUMBER"
+	    "unlimited"
+	}
+	test_gdb_complete_none \
+	    "$set_cmd 1"
 	test_gdb_complete_unique \
-	    "$set_cmd " \
+	    "$set_cmd u" \
 	    "$set_cmd unlimited"
     }