PR26665: relayfs-on-procfs megapatch

PR26665: relayfs-on-procfs megapatch

On platforms/configurations where debugfs is inaccessible (I'm
side-eyeing at you, secureboot + kernel_lockdown), the stap runtime
needs another way to hook up the relayfs / .cmd files to talk to
staprun/stapio in userspace.  kernel relayfs users all rely on
debugfs (tied closely to struct dentry*), and filesystems where
dentry*'s are not immediately available are SOL.

Until now.  This gigapatch forks pieces of runtime/transport/transport.c
into debugfs and procfs alternatives. The debugfs fork is just like
before. The procfs fork is new, and uses a proc_dir_entry <-> struct
path look-up table to map between procfs objects and the dentry*'s
that relayfs so loves.

The debugfs alternative is default, except when lockdown mode is
detected; then the runtime chooses procfs_p at the strategic moment.
stap -DSTAP_TRANS_PROCFS or -DSTAP_TRANS_DEBUGFS lets the user
override this heuristic.  (Going to a procfs default is worth
considering at some point.)

The staprun/stapio userspace is updated to search both
/sys/kernel/debug/systemtap and /proc/systemtap for the relay/.cmd
file endpoints.

Most of this gigapatch is moving code around in runtime/transport/ so
relay_v2 is agnostic to its enclosing filesystem, going through hooks
in transport.c to either procfs.c or debugfs.c.  The old
runtime/procfs.c file is stripped down to move common bits around a
little.

Signed-off-by: Frank Ch. Eigler <fche@redhat.com>