Deprecate windows-specific dll-symbols command and aliases

Deprecate windows-specific dll-symbols command and aliases

The "dll-symbols" command, specific to native Windows platforms,
gives the impression that the symbols were not loaded, first
because it completes silently, and second because the "info shared"
output does not get updated after the command completes:

    (gdb) dll-symbols C:\WINDOWS\syswow64\rpcrt4.dll
    (gdb) info shared
    From        To          Syms Read   Shared Object Library
    [...]
    0x77e51000  0x77ee2554  No          C:\WINDOWS\system32\rpcrt4.dll

(we exected the "Syms Read" column to read "Yes").

As far as I can tell, the symbols actually do get loaded, but completely
independently from the solib framework, which explains the silent
loading and the fact that the "Syms Read" column does not get updated.
See windows-nat.c::safe_symbol_file_add_stub, which calls symbol_file_add
instead of calling solib_add.

But, aside from the fact that the "Syms Read" status does not get
updated, I also noticed that it does not take into account the DLL's
actual load address when loading its symbols. As a result, I believe
that we get it wrong if the DLL does not get loaded at the prefered
address.

Rather than trying to fix this command, there does not seem to be
a reason other than historical for having Windows-specific commands
which essentially re-implements the "sharedlibrary" command. The
command interface is slightly different (the latter takes a regexp
rather than a plain filename), but it should be just as easy to use
the "sharedlibrary" command, or its "share" alias, as usisng the
"dll-symbols" command. For instance:

    (gdb) share rpcrt4.dll
    Reading symbols from C:\WINDOWS\system32\rpcrt4.dll...(no debugging symbols found)...done.
    Loaded symbols for C:\WINDOWS\system32\rpcrt4.dll
    (gdb) info shared
    From        To          Syms Read   Shared Object Library
    [...]
    0x77e51000  0x77ee2554  Yes (*)     C:\WINDOWS\system32\rpcrt4.dll

This patch therefore deprecates the "dll-symbols" command, as well
as its two aliases "add-shared-symbol-files" and "assf", with a view
of deleting them as soon as the 7.8 branch gets cut.

gdb/ChangeLog:

	* windows-nat.c (_initialize_windows_nat): Deprecate the
	"dll-symbols" command.  Turn the "add-shared-symbol-files"
	and "assf" aliases into commands, and deprecate them as well.
	* NEWS: Add entry explaining that "dll-symbols" and its two
	aliases are now deprecated.

gdb/doc/ChangeLog:

        * gdb.texinfo (Files): Document "add-shared-symbol-files"
        and "assf" as being deprecated.
        (Cygwin Native): Likewise for "dll-symbols".
        (Non-debug DLL Symbols): Remove reference to "dll-symbols"
        as a way to force the loading of symbols from a DLL.