rpcgen(1): default to "STDIN" as input file when compiling to headers.

rpcgen(1): default to "STDIN" as input file when compiling to headers.

FreeBSD, NetBSD, and GNU all use a variation of "STDIN" as the
default input filename when compiling to header files (-h) to prevent
a NULL-pointer deference. Adopt a similar approach.

ok millert@