unify the handling of string position arguments, fdo#75971 related

Only two text functions had the full set of checks as introduced by the
fix for fdo#75971. Let all text functions check their arguments in the
same way. Additionally this will ease a transition to accept string
lengths >64k in spreadsheet functions as now we have only one place that
checks for SAL_MAX_UINT16 instead of having that scattered all over the
place.

Change-Id: I454e617a59d0b3c2ca725047e7f8c7370bc0bb1f