[BUG][VM][FIX]file truncate primitive broken on *nix ( [er] Small fix needed in unix/vm/sqPlatformSpecific.h )

[tim at sumeru.stanford.edu]

**warning** This opinion is based on the copies of files I have from
SF/CVS as of a couple of days ago. This does not (sadly) mean that they
are the _right_ copies.

It appears to me that unix/vm/sqPlatformSpecific.h needs lines similar
to:-

#undef sqFTruncate
#define sqFTruncate(f,o) ftruncate(fileno(f), o)

adding in order to make us of the truncate call, Mac has it and
implements an ftruncate(), Win32 overrides the whole thing and RISC OS
leaves it null. Currently so does unix, which is why no truncation
occurs.