ExternalProcessExecutor spawned sub-processes with stderr and
stdout buffers, but did not read from those buffers. Sub-processes
producing output sometimes would fill up these buffers and then hang
until the timeout occurs and the process is killed.

This fixes #12839 (PDF conversion hangs for 10 seconds per page)
by properly discarding command output. The patch also deprecates an
outdated and dangerous API and offers a safer alternative. Splitting
command arguments based on whitespace may result in unwanted behavior
if an argument (e.g. a filename) contains whitespace.