patch 9.1.0535: newline escape wrong in ex mode

patch 9.1.0535: newline escape wrong in ex mode

Problem:  newline escape wrong in ex mode (Konrad Schwarz)
Solution: partly revert patch 7.3.014, remove backslash in front of a
          newline when not in prompt mode in ex line mode
          (Mohamed Akram)

This fixes newline escaping to allow passing multiple commands to
":global", multiple lines to shell commands, and ending lines in append
mode with backslashes. This should fix a POSIX/(traditional) VI
incompatiblity.

This reverts a previous incorrect attempt at patch v7.3.014 to fix
append mode which removed half of trailing backslashes which lead to,
eg. the following two commands being parsed as having a different number
of backslashes:

```
!echo foo\\\
```

```
!echo foo\\ \
```

fixes: #6135
fixes: #7244
closes: #15120

Signed-off-by: Mohamed Akram <mohd.akram@outlook.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>