function to handle the output. BUFFER may also be nil, meaning that
this process is not associated with any buffer.
-PROGRAM is the program file name. It is searched for in PATH. If
-nil, just associate a pty with the buffer. Remaining arguments are
-strings to give program as arguments.
+PROGRAM is the program file name. It is searched for in `exec-path'
+(which see). If nil, just associate a pty with the buffer. Remaining
+arguments are strings to give program as arguments.
If you want to separate standard output from standard error, invoke
the command through a shell and redirect one of them using the shell