PR23512: fix staprun/stapio operation via less-than-root privileges

PR23512: fix staprun/stapio operation via less-than-root privileges

Commit 7615cae790c899bc8a82841c75c8ea9c6fa54df3 for PR26665 introduced
a regression in handling stapusr/stapdev/stapsys gid invocation of
staprun/stapio.  This patch simplifies the relevant code in
staprun/ctl.c, init_ctl_channel(), to rely on openat/etc. to populate
and use the relay_basedir_fd as much as possible.  Also, we now avoid
unnecessary use of access(), which was checking against the wrong
(real rather than effective) uid/gid.