v3, 8 partsrefactor the filter process code into a reusable module

Refactor the filter.<driver>.process code into a separate sub-process module that can be used to reduce the cost of starting up a sub-process for multiple commands. It does this by keeping the external process running and processing all commands by communicating over standard input and standard output using the packet format (pkt-line) based protocol. Full documentation is in Documentation/technical/api-sub-process.txt.

This code is refactored from:

	Commit edcc85814c ("convert: add filter.<driver>.process option", 2016-10-16)
	keeps the external process running and processes all commands

Ben Peart (8):
  pkt-line: add packet_writel() and packet_read_line_gently()
  convert: Update convert to use new packet_writel() function
  convert: Split start_multi_file_filter into two separate functions
  convert: Separate generic structures and variables from the filter
    specific ones
  convert: Update generic functions to only use generic data structures
  convert: rename reusable sub-process functions
  sub-process: move sub-process functions into separate files
  convert: Update subprocess_read_status to not die on EOF

 Documentation/technical/api-sub-process.txt |  54 ++++++++++
 Makefile                                    |   1 +
 convert.c                                   | 159 +++++-----------------------
 pkt-line.c                                  |  31 ++++++
 pkt-line.h                                  |  11 ++
 sub-process.c                               | 120 +++++++++++++++++++++
 sub-process.h                               |  46 ++++++++
 7 files changed, 292 insertions(+), 130 deletions(-)
 create mode 100644 Documentation/technical/api-sub-process.txt
 create mode 100644 sub-process.c
 create mode 100644 sub-process.h