Add merge all option for name conflict resolution

Same as "merge", but remember the choice for future conflicts of
directories.

Thanks to willemw12.
Reduce code duplication in fileops:put_decide_cb()

Extract common piece to put_continue().
Merge branch 'add-merge-all-option-for-conflicts'

Add merge all option for name conflict resolution.  Same as "merge", but
remember the choice for future conflicts of directories.

Thanks to willemw12.
Fix documentation for :write

! isn't supported for a long time now.
Change mail extension to use <localleader>a key

Patch by filterfalse.
Remove part of manual page from vifm-app.txt

Patch by filterfalse :)
Made expand() expand environment variables

Thanks to filterfalse.
Fix tracking of files in trash on Windows

Thanks to filterfalse.
Do not show dead entries in :lstrash menu
Merge branch 'several-unrelated-adjustments'