Add functions to transform string to unsigned integer:

 * (xstrtoumax): from gnulib.
 * (parse_integer): from coreutils (dd.c).

Signed-off-by: Andrew Borodin <aborodin@vmail.ru>
Drop limit of edited file size.

Signed-off-by: Andrew Borodin <aborodin@vmail.ru>
Add threshold for file size.

Add "editor_filesize_threshold" ini option to ask open file if it size
is larger than specified threshold. Supported string value formats are:
"640000000", "64000K", "64M". Default value is 64M.

Signed-off-by: Andrew Borodin <aborodin@vmail.ru>