vim-patch:8.1.2053: SafeStateAgain not triggered if callback uses feedkeys()

Problem:    SafeStateAgain not triggered if callback uses feedkeys().
Solution:   Check for safe state in the input loop.  Make log messages easier
            to find. Add 'S' flag to state().

d103ee7843

Include misc1.c change from patch 8.1.2062.

Co-authored-by: Bram Moolenaar <Bram@vim.org>

runtime/lua/vim/_meta/vimfn.lua

@@ -8760,13 +8760,16 @@ function vim.fn.srand(expr) end
 --- added.  E.g, this checks if the screen has scrolled: >vim
 ---   if state('s') != ''
 ---
---- These characters indicate the state:
+--- These characters indicate the state, generally indicating that
+--- something is busy:
 ---     m  halfway a mapping, :normal command, feedkeys() or
 ---        stuffed command
 ---     o  operator pending or waiting for a command argument
 ---     a  Insert mode autocomplete active
 ---     x  executing an autocommand
----     c  callback invoked (repeats for recursiveness up to "ccc")
+---     S  not triggering SafeState
+---     c  callback invoked, including timer (repeats for
+---        recursiveness up to "ccc")
 ---     s  screen has scrolled for messages
 ---
 --- @param what? string