nvmf-autoconnect: Add option to specify path to systemctl

nvmf-autoconnect: Add option to specify path to systemctl

Many distros have adapted the usr merge and hence systemctl is install
in /usr/bin. Instead just hard coding the new path, let's introduce a
new build option to allow specifying the path to the systemclt
binary.

Note, the scripts also have hard coded paths to /bin/sh or
/bin/echo. None of these paths are covered by POSIX but in practice
all distros have those as symlinks when the usr merge have been
implemented. Let's address these issues when the pop up.

Signed-off-by: Daniel Wagner <dwagner@suse.de>