Give PROCESS the filter function FILTER; nil means no filter.
A value of t means stop accepting output from the process.
When a process has a filter, its buffer is not used for output.
Instead, each time it does output, the entire string of output is
passed to the filter.
The filter gets two arguments: the process and the string of output.
The string argument is normally a multibyte string, except:
- if the process' input coding system is no-conversion or raw-text,
it is a unibyte string (the non-converted input), or else
- if `default-enable-multibyte-characters' is nil, it is a unibyte
string (the result of converting the decoded input multibyte
string to unibyte with `string-make-unibyte'). (fn PROCESS FILTER)