Add savegame file path resolver to ClientServerCommandLineParser

[pakfront]

This moves the savegame file checks from DedicatedServer and MegaMekGUI into a common method on ClientServerCommandLineParser.
This new method fixes #4618, a common user error
-host and -dedicated args now both will look in ./savegames for a non-absolute file if it is not readable from ./
It also improves the error reporting by reporting the absolute paths of where it looked:
-dedicated ddmysave.sav,gz, which does not exist in this example, will report in the log:
Unable to read savegame file at "C:\Users\Natasha\Games\megamek\ddmysave.sav.gz" or "C:\Users\Natasha\Games\MegaMek\megamek\.\savegames\ddmysave.sav.gz"
This should work on macOS and Linux as well.