Added prototype for cram_set_header()
Fixed a couple complaints from clang.

1) Assigning hgetc() output to enumerated types doesn't work very well
unless I have the -1 case handled. (Technically this should perhaps be
EOF).

2) I wasn't checked the return value from hclose and hflush in all
situations. The hclose() call was in an error handler that is about to
return failure anyway so doesn't matter, but the hflush one needed
fixing.