BUG/MINOR: Fix search for -p argument in systemd wrapper.

BUG/MINOR: Fix search for -p argument in systemd wrapper.

Searching for the pid file in the list of arguments did not
take flags without parameters into account, like e.g. -de. Because
of this, the wrapper would use a different pid file than haproxy
if such an argument was specified before -p.

The new version can still yield a false positive for some crazy
situations, like your config file name starting with "-p", but
I think this is as good as it gets without using getopt or some
library.

Signed-off-by: Conrad Hoffmann <conrad@soundcloud.com>