Code from "conversion using external process" codepath has been
extracted to a separate sub-process.[ch] module.
* bp/sub-process-convert-filter:
convert: update subprocess_read_status() to not die on EOF
sub-process: move sub-process functions into separate files
convert: rename reusable sub-process functions
convert: update generic functions to only use generic data structures
convert: separate generic structures and variables from the filter specific ones
convert: split start_multi_file_filter() into two separate functions
pkt-line: annotate packet_writel with LAST_ARG_MUST_BE_NULL
convert: move packet_write_line() into pkt-line as packet_writel()
pkt-line: add packet_read_line_gently()
pkt-line: fix packet_read_line() to handle len < 0 errors
convert: remove erroneous tests for errno == EPIPE
ae7785de0e