Detect accidental use of smartd_warning script in '-M exec'.

smartd.cpp: Set SMARTD_SUBJECT to empty.
smartd_warning.sh.in, os_win32/smartd_warning.cmd: Abort if
SMARTD_SUBJECT is already nonempty.

git-svn-id: https://svn.code.sf.net/p/smartmontools/code/trunk@5403 4ea69e1a-61f1-4043-bf83-b5c94c648137