Maintenance release for v0.4.x series.

FIXES:

fd02e6351 #12701 ui: fix problem with sattr_T overflow
3e3002b90 #12710 fs: Ensure FileInfo struct is initialized
e4629f203 #12496 main.c: fix hang issue with recoverymode #12520
f558af8b1 #12209 fixed hang issue with --headless and -r option specified
cda119055 #12180 TUI: block signals on suspend
8399f7357 #12142 Fix issues revealed by gcc10 setting -fno-common by default
5e47cf27b netrw.vim: do not save +/* registers
5d41bfcce #11907 quickfix.c: Fix vimgrep regression
641e22913 #11903 screen: add missing redraws after a message
47bd62c15 loop_close: close all handles
d2730365e loop_close: call uv_stop(), fix bug
284b398eb #11821 loop_close: timout after 2 seconds
1b2666cd2 messages: echo "line1\r\nline2" should not clear line1
1bf90bf9d #11651 API: fix crash on copy_object(kObjectTypeWindow)
fafd63626 #11566 libcallnr: Use int, not int64_t, as the return type for Vim compat
972dd758c #11460 win_line: Fix crash with 'rightleft' in :terminal
4c48cf302 f_getenv/setenv: Access v_special when v_type is VAR_SPECIAL
2cd1ff762 #11360 paste: Select-mode, Visual-mode

Problem:    Empty group in 'tabline' may cause using an invalid pointer.
Solution:   Set the group start position. (closes vim/vim#6505)
https://github.com/vim/vim/commit/f56c95fdad5af521887f8cd7bc15729b5355231d
(cherry picked from commit 9443d3994190943c2b9d8c4efefcf3de854ec7be)

sattr_T was defined as uint16_t. But this is not enough to handle the
24-bit colors of the terminal. To solve this problem, change it to int.
In 32bit, int may overflow. So, if it overflows, change it to ignore it
without adding more attr_entries.

fixes #12366

(cherry picked from commit 872ecf65d10f0d22bbf3e2257f9ae89a6c61d2f4)

travis: Use Python 3.7.1

Zero out the caller's FileInfo in all the functions which populate the
struct.  The contents are considered private, so we need to ensure
it's initialized.

If the stat call fails, the buffer we get back from libuv may not have
any valid data in it, so don't copy it into the caller's FileInfo.

This was happening, expectedly, in functional/ex_cmds/write_spec.lua's
"write errors out correctly", which caused it to fail in certain
environments:

    test/functional/ex_cmds/write_spec.lua:130: Expected objects to be the same.
    Passed in:
    (string) 'Vim(write):E212: Can't open file for writing: not a directory'
    Expected:
    (string) 'Vim(write):E166: Can't open linked file for writing'

In the case of recoverymode, the headlessmode was true, causing the UI
to hang without starting. Fix this problem by setting headlessmode to
true for List swap files only.

Co-authored-by: erw7 <erw7.github@gmail.com>