[mkgmap-dev] Commit r4601: implement and document new parameter check for --generate-sea

Version mkgmap-r4601 was committed by gerd on Fri, 12 Feb 2021 implement and document new parameter check for --generate-sea - sea4.patch by Mike Baggaley with small changes - perform more checks if parameter is used - don't perform checks if parameter is not used - minor code changes to fix typos in method names http://www.mkgmap.org.uk/websvn/revision.php?repname=mkgmap&rev=4601