gdb: convert linux-namespaces debug to the new(er) debug scheme

gdb: convert linux-namespaces debug to the new(er) debug scheme

Convert 'set debug linux-namespaces' to the new(er) debug scheme.  As
part of this change I converted the mnsh_debug_print_message function,
which previously printed its output, to instead return a std::string,
this string is then printed using linux_namespaces_debug_printf.  The
mnsh_debug_print_message function is only used as part of the debug
output.

I also updated one place in the code where debug_linux_namespaces, the
debug control variable, which is a boolean, was assigned an integer.

When debug is turned on then clearly the output is now different, but
in all other cases, there should be no user visible change in GDB
after this commit.

Approved-By: Tom Tromey <tom@tromey.com>