Start a program in a subprocess. Return the process object for it.
NAME is name for process. It is modified if necessary to make it unique.
BUFFER is the buffer (or buffer name) to associate with the process.
Process output goes at end of that buffer, unless you specify
an output stream or filter function to handle the output.
BUFFER may be also nil, meaning that this process is not associated
with any buffer
COMMAND is the shell command to run.
An old calling convention accepted any number of arguments after COMMAND,
which were just concatenated to COMMAND. This is still supported but strongly
discouraged.