Very small style change in commands.c:split_screen
Check whether we are running inside screen
Fix black screen on :shell after :screen w/o it
Update documentation

State that %s macro is ignored unless vifm is running inside screen.
Merge branch 'check-running-in-screen'
Rename NUM_SORT_OPTIONS -> LAST_SORT_OPTION

Also add curly brackets in blocks where renaming was done.
Add NO_SORT_OPTION to use as unset option value
Add SORT_OPTION_COUNT value

And use it instead of LAST_SORT_OPTION where appropriate semantically.
Use memset() to fill FileView::sort array

Instead of handmade loops.
Simplify outputting of sort options
Move sort info writing to a separate function
Rename get_sort{,_info}, make it less error prone

Thanks to Christoph (informationen)
Add get_pid() utility function

It gets identification of the current process in a portable way.
Use absolute values of sort options in comparisons

Otherwise they will catch only one of two cases.
Fixed races on writing vifminfo file

This updating of vifminfo file is done in three steps:
 1) copy vifminfo
 2) update its copy
 3) replace original file with its modified copy

Thanks to Christoph (informationen)
Merge branch 'vifminfo-fixes'