copy, tee: assume EINTR is always defined: remove #ifdefs

copy, tee: assume EINTR is always defined: remove #ifdefs

Don't use "#ifdef EINTR".  dd.c has been doing that since 2004.
* src/copy.c (sparse_copy): Remove #ifdef...#endif around EINTR use.
* src/tee.c (tee_files): Remove #ifdef...#endif around EINTR use.
If we need it, add something like this in system.h:
/* When EINTR is not defined, define it to an improbable value
   so that each use does not have to be #ifdef'd.  */
 #ifndef EINTR
 # define EINTR 999988
 #endif