Merge branch 'bp/sub-process-convert-filter'

Merge branch 'bp/sub-process-convert-filter'

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