mdl_import: IncludePhy default, add file validation

Most people won't need the phy so "Include Phy" now defautls to off. I
also added a little file validation check that won't write the file if
it doesn't contain strings known for those filetypes to always contain
(IDST, IDSV, VPHY, etc). Unfortunately I can't find such a string in vtx
files so those aren't being checked at this time (if anyone knows
something like that that is always present in VTX files, please let me
know and I'll add it to the validation check table).