coverity/13695: Unchecked return value: RI.

Problem    : Unchecked return value (CHECKED_RETURN) @ 8554.
Diagnostic : Real issue.
Rationale  : Other invocations of `do_source` are checked and generate
             an error message if fail. There seems to be no reason why
             this particular instance could not fail the same.
Resolution : Check invocation and generate error message on failure.
coverity/13696: Unchecked return value: RI.

Problem    : Unchecked return value (CHECKED_RETURN) @ 2644.
Diagnostic : Real issue.
Rationale  : Other `u_save` invocations are checked, and there's no
             reason to think this invocation could not fail.
Resolution : Check and return if failed (other previous checks in the
             same function just return, without reporting error, so
             we just do the same).
Passing-by: Fix FALSE/FAIL confusion.

FALSE was being used instead of FAIL.
They happen to have the same value, so it works the same.
But from function comment it's clear it uses the OK/FAIL convention.
coverity/75594: Explicit null dereferenced: RI.

Problem    : Exlicit null dereferenced (FORWARD NULL) @ 2859.
Diagnostic : Real issue.
Rationale  : Code within `if (!p_bk)` seems to assume `backup` not null
             at that point, which may not be true.
Resolution : Don't enter conditional on null `backup`.
Merge pull request #1688 from elmart/coverity-issues-1

coverity issues
Use stdbool: skip_redraw
Use stdbool: do_redraw
Use stdbool: ignore_script
Use stdbool: msg_scrolled_ign
Use stdbool: redraw_cmdline
Use stdbool: clear_cmdline

Change need_clear to bool as well, since it was set to clear_cmdline.
Use stdbool: mode_displayed
Use stdbool: cmdline_star
Use stdbool: exec_from_reg
Use stdbool: compl_interrupted

An instance in eval.c wasn't dealt with, as eval.c will be dropped in
the future.
Use stdbool: screen_cleared

If you're reading this from a PR, then I've forgotten to ask about the
change in logic I made in screen.c necessitated by using a bool, and how
I'm not completely sure if it doesn't change semantics.
Use stdbool: keep_msg_more
Use stdbool: need_fileinfo
Use stdbool: msg_scroll

Also standardize the name used for temporarily storing msg_scroll,
calling it 'save_msg_scroll'.
Use stdbool: msg_didout