fsx: add optional logid prefix to log messages

fsx: add optional logid prefix to log messages

When redirecting the intermixed output of several fsx processes
to a single output file, it is usefull to prefix debug log messages
with a log id. Use fsx -j <logid> to define the log messages prefix.

Fix implementation of prt() function to avoid using a temp buffer
and convert some more printf() calls to use ptr() instead.

Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>