Problem:    Source command doesn't check for the sandbox. (Armin Razmjou)
Solution:   Check for the sandbox when sourcing a file.
https://github.com/vim/vim/commit/53575521406739cf20bbe4e384d88e7dca11f040

Fixes https://github.com/neovim/neovim/issues/9892

gcc-9 has [improved compliance] with the C spec for lifetime of compound
literals, tying their lifetime to block scope instead of function scope.
This makes the behavior comparable to all other automatic variables.

Using the SHM_ALL #define instantiated a compound literal local to an if
clause and assigned the address to a "char_u *".  Since the pointer was
then being used outside of the if clause, it was using an invalid
address.

[improved compliance]: https://gcc.gnu.org/gcc-9/porting_to.html#complit

Closes #9855

crash reported in #9584

Problem:    Look-behind match may use the wrong line number. (Dominique Pelle)
Solution:   Use the line number in regsave instead of the one in behind_pos,
            we may be looking at the previous line. (closes vim/vim#3749)
https://github.com/vim/vim/commit/866f3558141aa68862aa2fedbb7747bf2365e838

Reverts d2944e6a298e. mf_open() _can_ fail if the file does not exist.

closes #9503
closes #9504

Move `has_eval_provider()` check to `eval_call_provider()` to make sure that
every code path calls it first.

Previously we would, when pynvim was missing, get a nice error message for
`:python3 1`, but not for `:py3file blah`.

Fixes https://github.com/neovim/neovim/issues/9485

Apparently Konsole's terminfo is still broken.

ref #9364
closes #9420