Send current contents of region as input to PROCESS.
PROCESS may be a process, a buffer, the name of a process or buffer, or
nil, indicating the current buffer's process.
Called from program, takes three arguments, PROCESS, START and END.
If the region is more than 500 characters long,
it is sent in several bunches. This may happen even for shorter regions.
Output from processes can arrive in between bunches. (fn PROCESS START END)