MFC After:	1 week

MFC atfer:	1 week

It includes improvements in the libreadline compatibility and a change
from pstef@ which fixes filename autocompletion for strings like a\)b

It contains changes pushed by pstef@ when working on improving the
completion for /bin/sh

On arm and arm64, where chars are unsigned by default, buildworld dies
with:

--- terminal.o ---
/usr/src/contrib/libedit/terminal.c:569:41: error: comparison of
integers of different signs: 'wint_t' (aka 'int') and 'wchar_t' (aka
'unsigned int') [-Werror,-Wsign-compare]
                                     el->el_cursor.v][where & 0370] !=
                                     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ^
/usr/src/contrib/libedit/terminal.c:659:28: error: comparison of
integers of different signs: 'wint_t' (aka 'int') and 'wchar_t' (aka
'unsigned int') [-Werror,-Wsign-compare]
                                     [el->el_cursor.h] == MB_FILL_CHAR)
                                     ~~~~~~~~~~~~~~~~~ ^  ~~~~~~~~~~~~

Fix this by making MB_FILL_CHAR a wint_t, so no casting is needed.

Note that in https://reviews.freebsd.org/D21584 this was also proposed
by Yuichiro Naito <naito.yuichiro_gmail.com>.

Reviewed by:	bapt
Subscribers:	naito.yuichiro_gmail.com, ml_vishwin.info
MFC after:	3 weeks
X-MFC-With:	r352275
Differential Revision: https://reviews.freebsd.org/D21657

This function is not needed anymore, it allows old sh binary to continue
to run and avoid breaking backward compatibility.
Note that is now just calls the regular _el_fn_complete which does a proper
job at quoting.

Discussed with:	jilles