library: normalize recently added namespaces interface

library: normalize recently added namespaces interface

While 'invisible' thread subdirectories are accessible
under /proc/ with stat/opendir calls, they have always
been treated as non-existent, as is true with readdir.

This patch trades the /proc/#/ns access convention for
the more proper /proc/#/task/#/ns approach when thread
access is desired. In addition some namespace code has
been simplified and made slightly more efficient given
the calloc nature of proc_t acquisition and its reuse.

Reference(s):
commit a01ee3c0b32d4c39aa83066ed61103343469527e

Signed-off-by: Jim Warner <james.warner@comcast.net>