split: accept new output --filter=CMD option

split: accept new output --filter=CMD option

* src/split.c: Include <signal.h>, <sys/wait.h> and "sig2str.h".
(FILTER_OPTION): New anonymous enum member.
(filter_command, filter_pid): New globals.
(open_pipes, open_pipes_alloc, n_open_pipes): Likewise.
(oldblocked, newblocked): Likewise.
(longopts): Add "filter".
(usage): Document --filter.
(create): Extend to create a pipe and fork "sh -c CMD".
(closeout): Adapt to close a pipe and wait for child process.
(cwrite): Call closeout, not just close.
(lines_chunk_split): FIXME
(bytes_chunk_extract): FIXME
(opid, ofile_open, lines_rr, main): FIXME
(ignorable): New function, to encapsulate EPIPE test.