Respect RLIMIT_FSIZE when extending a file via truncat(2)/ftruncate(2).

Respect RLIMIT_FSIZE when extending a file via truncat(2)/ftruncate(2).
This refactors the commin parts of sys_truncate() and sys_ftruncate()
into dotruncate().  If the new size of the file is larger than the
RLIMIT_FSIZE limit _and_ the file is being extended, not truncated,
return EFBIG.  Adapted from a diff by Piotr Durlej.
With help from and OK by deraadt@ guenther@.