gdb, gdbserver: extend RSP to support namespaces

gdb, gdbserver: extend RSP to support namespaces

Introduce a new qXfer:libraries-svr4:read annex key/value pair

    lmid=<namespace identifier>

to be used together with start and prev to provide the namespace of start
and prev to gdbserver.

Unknown key/value pairs are ignored by gdbserver so no new supports check
is needed.

Introduce a new library-list-svr4 library attribute

    lmid

to provide the namespace of a library entry to GDB.

This implementation uses the address of a namespace's r_debug object as
namespace identifier.

This should have incremented the minor version but since unknown XML
attributes are ignored, anyway, and since changing the version results in
a warning from GDB, the version is left at 1.0.