Fix filtering directories on zf command

Thanks to filterfalse.
Add empty lines to filetype.h

For better readability.
Introduce filelist:iter_selected_entries()

Provides much more convenient interface of going through selected
entries of a file list comparing to manual looping over the array.
Move VIFM_PSEUDO_CMD define to the top of filetype
Better code in running:run_file()
Fix :fileviewer directory pattern matching

After removing trailing slash.

Thanks to filterfalse.
Style corrections for quickview:quick_view_file()
Fix directory matching & get_all_programs_for_file

After removing trailing slash.

Thanks to filterfalse.
Fix get_default_program_for_file() for dirs

After removing trailing slash.

Thanks to filterfalse.
Move typed fname functions in filelist
Fix typo in filetype.c comment
Style improvements for cfg/info:write_assocs()
Fix merging of file associations

Not sure they ever worked as expected.  Not sure it will work now,
but new code has more chances.

This isn't very important as it's more like obsolete feature that is
discouraged and no one complained that it doesn't work.
Small filetype style/comments update
Merge branch 'fix-more-trailing-slash-regressions'

Fix trailing slash regression for zf/:filetype/:fileviewer.

Thanks to filterfalse.