isag: Add support for saYYYYMMDD log files format

isag: Add support for saYYYYMMDD log files format

The fill_file_menu() function was rewritten and tries to handle
all three possible formats (`saYYYYMMDD, `saDD', `YYYYMM/saDD')
by collecting existing log files into a temporary list of strings,
which is then used after being sorted to generate the menu.

NOTE: isag is no longer maintained. This patch has been provided by
Debian team (see #146).

Signed-off-by: Sebastien GODARD <sysstat@users.noreply.github.com>