move the bulk of [task_]ns_* tapsets to helper fns

move the bulk of [task_]ns_* tapsets to helper fns

runtime/linux/namespaces.h: define helper fns to get specific pid or user
information based on the target namespaces. rely on the new runtime
variable _stp_namespace_pid rather than the _STP_NAMESPACE_PID macro
runtime/transport/transport.c: define runtime var _stp_namespace_pid
that will contain the target namespaces pid. default to _stp_pid
if there was no target ns pid specified
tapset/linux/*.stp: replace body of [task_]ns_* tapset functions with a
call to the helper functions in namespaces.h
translate.cxx: remove _STP_NAMESPACE_PID macro definition