This also makes shebang always use env command, and drops unnecessary
'bash -c' or 'sh -c' when a signle command is invoked in the shell,
like sleep or echo.
This applies the existing SocketUser=/SocketGroup= options to units
defining a POSIX message queue, bringing them in line with UNIX
sockets and FIFOs. They are set on the file descriptor rather than
a file system path because the /dev/mqueue path interface is an
optional mount unit.