Improve argument passing in Jetty start/stop scripts

# ----------------------------------------------------------------------------------------------------------------
# Optional ENV vars
# -----------------
#   XWIKI_OPTS - parameters passed to the Java VM when running XWiki e.g. to increase the memory allocated to the
#       JVM to 1GB, use set XWIKI_OPTS=-Xmx1024m
#   JETTY_PORT - the port on which to start Jetty.
#   JETTY_STOP_PORT - the port on which Jetty listens for a Stop command.
#
# Optional Parameters
# -------------------
#   -p, --port: The Jetty HTTP port to use. Overrides any value from JETTY_PORT. Defaults to 8080.
#   -sp, --stopport: The Jetty stop port to use. Overrides any value from JETTY_STOP_PORT. Defaults to 8079.
#   -ld, --lockdir: The directory where the executing process id is stored to verify that that only one instance is
#       started. Defaults to /var/tmp.
#   -k, --kill: If set then kills any already executing XWiki instance before starting a new one.
#
# Example
# -------
#   start_xwiki.sh -p 8080 -sp 8079 -k
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