Don't pass possible command escapes to the 'mail' command (#1535).

The 'mail' command from GNU mailutils < 3.13 processes '~! COMMAND'
even if used non-interactively (https://savannah.gnu.org/bugs/?60937).

smartd.cpp: Sanitize device identify information which is passed to
SMART_DEVICEINFO environment variable and written to syslog.

smartd_warning.sh.in: Abort script if a message line begins with a
possible command escape.

Thanks to 0x3l leox14@protonmail.com for reporting this security issue.

git-svn-id: https://svn.code.sf.net/p/smartmontools/code/trunk@5238 4ea69e1a-61f1-4043-bf83-b5c94c648137