Implement mount namespace support for native Linux targets

Implement mount namespace support for native Linux targets

This commit allows GDB to access executables and shared libraries
on native Linux targets where GDB and the inferior have different
mount namespaces.

gdb/ChangeLog:

	* linux-nat.c (nat/linux-namespaces.h): New include.
	(fileio.h): Likewise.
	(linux_nat_filesystem_is_local): New function.
	(linux_nat_fileio_pid_of): Likewise.
	(linux_nat_fileio_open): Likewise.
	(linux_nat_fileio_readlink): Likewise.
	(linux_nat_fileio_unlink): Likewise.
	(linux_nat_add_target): Initialize to_filesystem_is_local,
	to_fileio_open, to_fileio_readlink and to_fileio_unlink.
	(_initialize_linux_nat): New "set/show debug linux-namespaces"
	commands.
	* NEWS: Mention new "set/show debug linux-namespaces" commands.

gdb/doc/ChangeLog:

	* gdb.texinfo (Debugging Output): Document the "set/show debug
	linux-namespaces" command.