[WIP] screen_puts_len: Ignore control chars.

[justinmk]

TODO:

• tests
• handle tab chars "\t" Incorrect formatting in output when run using '!' inside nvim [Not in vim]. #2958 !cat shows tabs as 1 space #5100 (fixed by [RFC] shell: use msg_outtrans_len_attr for :!cmd #7844)
• handle bell chars "\x07" Zero width control characters insert spaces with shell ex commands #4338 (with [RFC] shell: use msg_outtrans_len_attr for :!cmd #7844, bell chars are printed as ^G)
• investigate relation to grep is broken I think? #1776 (conclusion: unrelated)

If a binary or other output is read from :! the screen should not be
affected by accidental (or intentional) control chars.
Legacy Vim :! gets scrubbed (possibly by canonical ("cooked")
mode in mch_call_shell(), see settmode(TMODE_COOK)).

Expected behavior:

:!echo -en '\x08'|cat -             " empty (except "Press ENTER")
:!echo -en '\x08'                   " empty (except "Press ENTER")
:!echo -en '\n'                     " extra line
:echo system('echo -en ''\x08\n''') " ^H

Issue #4338
:!echo -en '\x07\x07\x07\x07text'   " 'text' (not '    text')

Issue #2958 (Vim tputs()?)
:!echo -e "1\t2\t3"                 " '1       2       3'

Closes #5442 closed by #6618
Closes #4142 closed by #6618
Closes #2958 [WIP]
Closes #4338 [WIP]

[justinmk]

fixed by #7844

tests and 'writedelay' addressed by by #6713