MFC r263775:

Avoid "cc1: warning: is shorter than expected" when using GNU cpp in
combination with dtrace scripts, which have "#!/usr/sbin/dtrace -Cs"
shebang lines.  This is because dtrace positions the file pointer after
the shebang line, before passing the file to GNU cpp.

To fix the warning, adjust the size downwards by the current position,
after a bit of sanity checking.

Suggested by:	avg