misc/mc: the port had been improved (+)

• GC outdated CONFLICTS (misc/mc-light had been removed on 2019-10-16)
• Add missing SCREEN_DESC (screen library) option description
• Offer new option, EXTATTR (ext2fs extended attributes support) and ensure, in somewhat ugly way, that dependency on `sysutils/e2fsprogs' won't be silently picked up if this option is disabled
• Tell users that /bin/sh, our base system sh(1), is not supported as concurrent subshell because it does not allow "precmd" hooks [1]
• Fix entering RAR archives after archivers/unrar had been updated to version 6.x [2]
• Use helper target and simplify installation of documentation files

PR: 208391 [1], 251404 [2]