Introduce appendable long strings in parser for MGM and CPC protocol
keeping the maximum line size to 512 bytes.

Use LongString for CPC protocol 'define process' arguments 'env' and 'args'.

These long strings are sent as:
argname:"first-part-of-data..."\n
+argname:"second-part-of-data..."\n
...
+argname:"last-part-of-data..."\n

Sending lines starting with '+' to old clients will make CPCD not defining
process, in same way as specifing unknown argument.

The use of quotes is to disable triming white spaces since they need to be
kept while concatenating the parts.  Using quotes is an old feature.