coverity/71507: Unchecked return value (CHECKED_RETURN)

Don't check for err->set since object_to_vim always return false when error is set
fix #1027: :wundo segfault in new, non-empty buffer
input/job: process ctrl+c and do conversion in the read callback

- Extract `process_interrupts` out of `convert_input`
- Instead of waiting for os_breakcheck/os_inchar calls, call `convert_input`
  and `process_interrupts` directly from the read callback in input.c.
- Remove the `settmode` calls from `job_wait`. Now that interrupts are
  processed in the event loop, there's no need to set the terminal to cooked
  which introduces other problems(ref 7.4.427)
input: Fix sizes of input/read buffers

Input buffer must be bigger than read buffer to ensure it always has space for
converted data.
api: Add vim_input function and mark vim_feedkeys as deferred

The `vim_feedkeys` must be deferred because it can potentially free the buffer
passed to `os_inchar`(which in turns calls `vim_feedkeys` indirectly).

The new `vim_input` function can be used to emulate user input(Since it does not
mess with the typeahead, it is safe to execute without deferring).
travis: Make sure valgrind report is shown on test failures
Merge PR #1357 'Fixes to input/job modules'
fs.c: add FUNC_ATTR_NONNULL_ALL to all functions
CONTRIBUTING.md: more guidance on cosmetic changes #1346
Merge pull request #1312 from GokuITA/coverity1289

coverity/71507: Unchecked return value (CHECKED_RETURN)
Mark patch 427 as included. (#1357)
Merge pull request #1361 from splinterofchaos/silent-system

Mark patch 427 as included.
unittest: fix handling of pragma pack in `cimport`
unittest: increase number of retries in cimport
Merge pull request #1349 from stefan991/fix-cimport-pragma-pack

Fix handling of `#pragma pack` in `cimport` lua helper
CONTRIBUTING.md: Add link to CLA.
Fix python setup: Report errors.

No error indication was being given if something went wrong while
setting up python.
Fix python setup: Make sure python version on user's path is picked.

Problem:  executable() was detecting python on user's path, but
          system() was executing system-level python.

Solution: Make sure python version on user's path is executed.
          Use exepath() to force system() to do so.
Merge PR #1358 'Fix python setup'
Merge pull request #1364 from fwalch/cla

CONTRIBUTING.md: Add link to CLA.