Option -Zm: Enable Masm v5 Compatibility

Option -Zm (or setting OPTION M510) will do:

- set OPTION OLDSTRUCTS

- set OPTION DOTNAME

- set OPTION SETIF2:TRUE

- set OPTION OFFSET:SEGMENT (if no model is set)

- set OPTION NOSCOPED (if no model with language specifier is set)

- allow to define data items behind code labels

- allow "invalid" use of REP/REPE/REPNE instruction prefixes

- change precedence of [] and () operator from 1 to 9. Hence expression -5[bx] is parsed as (-5)[bx], while without -Zm it is parsed as -(5[bx]), which generates an error.

Other Masm v5.1 compatibility options aren't implemented yet.