use pkg-config to look for libidn if available.

checking <idn/idna.h> rather than including -I/usr/include/idn for Solaris 11.

closes #3624
remove PKG_CHECK_MODULE() check and always use AC_CHECK_HEADERS()

only check for <idn/idna.h> when <idna.h> isn't found

closes #3626
do not explicitly reset the terminal attributes and move the cursor in mutt_endwin().  endwin() itself should handle this.

closes #3627
add support for $mail_check_recent to mh mailbox driver

closes #3629
do not use mutt_message() to display prompt in mutt_yesorno().

properly calculate screen width of prompt string by using mutt_strwidth() rather than strlen()

use safe_asprintf() return the formatted yes/no string since specifying COLS+1 is wrong when dealing with multibyte characters

closes #3412
closes #3352