gdb: Introduce user-friendly namespace identifier for "info shared"HEADmaster

GDB has had basic support for linkage namespaces for some time already,
but only in the sense of managing multiple copies of the same shared
object being loaded, and a very fragile way to find the correct copy of
a symbol (see PR shlibs/32054).

This commit is the first step in improving the user experience around
multiple namespace support. It introduces a user-friendly identifier for
namespaces, in the format [[<number>]], that will keep consistent between
dlmopen and dlclose calls. The plan is for this identifier to be usable
in expressions like `print [[1]]::var` to find a specific instance of a
symbol, and so the identifier must not be a valid C++ or Ada namespace
identifier, otherwise disambiguation becomes a problem. Support for
those expressions has not been implemented yet, it is only mentioned to
explain why the identifier looks like this.

This syntax was chosen based on the C attributes, since nothing in GDB
uses a similar syntax that could confuse users. Other syntax options
that were explored were "#<number>" and "@<number>". The former was
abandoned because when printing a frame, the frame number is also
printed with #<number>, so in a lot of the context in which that the
identifier would show up, it appears in a confusing way. The latter
clashes with the array printing syntax, and I believe that the having
"@N::foo" working completely differently to "foo@2" would also lead to a
bad user experience.

The namespace identifiers are stored via a vector inside svr4_info
object. The vector stores the address of the r_debug objects used by
glibc to identify each namespace, and the user-friendly ID is the index
of the r_debug in the vector. This commit also introduces a set storing
the indices of active namespaces. The glibc I used to develop this patch
(glibc 2.40 on Fedora 41) doesn't allow an SO to be loaded into a
deactivated namespace, and requesting a new namespace when a namespace
was previously closed will reuse that namespace. Because of how this is
implemented, this patch lets GDB easily track the exact namespace IDs
that the inferior will see.

Finally, two new solib_ops function pointers were added, find_solib_ns
and num_active_namespaces, to allow code outside of solib-svr4 to find
and use the namespace identifiers and the number of namespaces,
respectively. As a sanity check, the command `info sharedlibrary` has
been changed to display the namespace identifier when the inferior has
more than one active namespace. With this final change, a couple of tests
had to be tweaked to handle the possible new column, and a new test has
been created to make sure that the column appears and disappears as
needed, and that GDB can track the value of the LMID for namespaces.

Approved-by: Kevin Buettner <kevinb@redhat.com>

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