Reimplement DAP's stopAtBeginningOfMainSubprogram

Right now, stopAtBeginningOfMainSubprogram is implemented "by hand",
but then later the launch function uses "starti" to implement
stopOnEntry.  This patch unifies this code and rewrites it to use
"start" when appropriate.

Reviewed-by: Kévin Le Gouguec <legouguec@adacore.com>