Fix shell command parsing

Valid shell commands of the form

command_line VAR='VALUE' /bin/something

were not properly parsed by runcmd.c.

This caused issues with check_mk's flexible notifications.

This patch fixes the issue by detecting an '=' character before the first space in a command and forcing the use of a command shell to process it, rather than using execv

See http://tracker.nagios.org/view.php?id=582 for the full details