syntax: bash launcher script hardening

[hraban]

Apply various hardening idioms to the default Bash launcher script:

• quote all variables
• quote all path names
• explicit error message on wrong usage
• use exec for the "wrapped" command
• use common hardening flags (this one is not technically useful in this script as it is, but it's a good idea anyway, in case it is ever modified with a statement that could benefit from it)
• use /usr/bin/env