full-trace.sh: check if uprobes exist before enabling them

Currently, the full-trace.sh script always enables uprobes tracing,
even if no uprobes have been planted. This results in an error, as
the uprobes subsystem has not been enabled and the /tmp/uprobes
file does not exist. This patch introduces checks on the existence
of the affected files before any command touches them. In case no
uprobe has been planted, the uprobes_on() function will print a
warning message to standard output.

Reported-by: Serena Ziviani <senseriumi@gmail.com>
Signed-off-by: Serena Ziviani <senseriumi@gmail.com>
Signed-off-by: Arianna Avanzini <avanzini.arianna@gmail.com>
full-trace.sh: reformat of awk scripts

Reformat of the awk parts in remove-spurious-uprobes() and
add-userspace-functions-duration() functions.

Signed-off-by: Serena Ziviani <senseriumi@gmail.com>