Add FILE_RETRY_ERRNO() definition, to be used in sopen()/lock() loops

Note: EBUSY was added to the list of errno values that should cause a retry
as observed on Vertrauen, running sbbs-linux on a Samba mount, opening
locked files can return failure (-1) with errno = EBUSY, e.g.
evnt BBS Events !ERROR 16 (Device or resource busy) in logon.cpp line 616 (logonstats) reading "system stats" access=0

This was apparently resolved by adding EBUSY (16) the values that trigger an sopen()
retry (i.e. from [f]nopen()).