[FIX JENKINS-22818] fix jenkins restart on Linux after plugin update if the java executable running Jenkins is started without its full path

The patch uses 'execvp' instead of 'execv' to lookup the java executable filename in the PATH if the specified filename does not contain a slash (/) character.
Regression introduced in 547d4ed8d35de6be4b2788e4816019bb8cc94902
Changelog for PR #1239