Modified text and menu files can now be placed in mods/text/*

(e.g. mods/text/menu/main.msg), but only when the mods dir is configured in
SCFG->System->Advanced Options (by default, this set to ../mods/).

While menu-related functions will always check/read the file from the
mods/text dir (if exists), printfile() function requires the new P_MODS flag
to be specified to check/read the file from the mods/text directory.

This resolves feature request/issue #879