Use background task in delete_file_bg()
Use background task in cpmv_files_bg()
Do not expose background task details

No need now, as they are better encapsulated after recent changes.
Rename delete_file% to delete_files%

Those functions actually removes set of files.
Remove job field from fileops.c:bg_args_t

It's not used anymore.
Remove unused job argument from bg_task_func
Merge branch 'use-new-bg-funcs-in-fileops'

Reuse new code from background.c in fileops.c.
Ignore trailing whitespace after " &" on cmd-line

Example of previously ill-formed command:

    :mv & | jobs

It failed with "Trailing characters" error because of trailing space
after the ampersand.  Something simpler used to fall to (note
trailing spaces):

    :delete &
Merge branch 'ignore-trailing-whitespace-after-bg-mark'

Ignore trailing whitespace after background mark (" &") in command-line
mode.