[nto] Fixes for nto procfs.

[nto] Fixes for nto procfs.

Fix errnoeus construction of procfs path. The issue is, after the first
info pidlist or first run, the path for local node (the most common node)
will be reset to empty which makes subsequent queries and runs impossible.

gdb/ChangeLog:

	* nto-procfs.c (nto_procfs_path): Rename to...
	(nodestr): ... this, and change type.
	(nto_node): Use new variable and logic accordingly.
	(procfs_open_1): Use new variable name. Use local buffer to construct
	procfrs path.
	(procfs_pidlist): Use NODESTR to construct procfs path.
	(procfs_files_info): Use NODESTR to output meaningful text.
	(do_attach): Construct procfs using NODESTR.
	(procfs_create_inferior): Compare pointer to NULL.