ui: add a configwith method This is a long-overdue generalization of the pattern in configint and configbool.

py3: convert the mode argument of os.fdopen to unicodes (2 of 2)

py3: convert the mode argument of os.fdopen to unicodes (1 of 2) os.fdopen() does not accepts bytes as its second argument which represent the mode in which the file is to be opened. This patch makes sure unicodes are passed in py3 by using pycompat.sysstr().