PostgreSQL: changing data directory location (sshfs)
I finally solved the problem using -o allow_other when mounting the /mnt/titanBDD with sshfs.

The error message was still :


  Error: could not exec

/usr/lib/postgresql/9.4/bin/pg_ctl /usr/lib/postgresql/9.4/bin/pg_ctl 
start -D /var/lib/postgresql/9.4/redirect_TITAN
-l /var/log/postgresql/postgresql-9.4-main.log -s -o -c
config_file="/etc/postgresql/9.4/main/postgresql.conf"



(which repeat the command twice?!)

when executing:

usr/lib/postgresql/9.4/bin/pg_ctl 
start -D /var/lib/postgresql/9.4/redirect_TITAN
-l /var/log/postgresql/postgresql-9.4-main.log -s -o -c
config_file="/etc/postgresql/9.4/main/postgresql.conf"


but I noticed the user postgres didn't have access to the mounted point which put me on the track.

Now PostgreSQL starts without any problem.