Previously, it was not possible to use
sha1sum test.c | sha1sum -c
because the program would not differenciate between an empty
argument and a non-specified argument.
Moreover, why not allow this?
sha1sum -c hashlist1 hashlist2
Digging deeper I found that using function pointers and a
modification in the crypt-backend might simplify the program
a lot by passing the argument-list to both cryptmain and
cryptcheck.
Allowing more than one list-file to be specified is also
consistent with what the other implementations support,
so we not only have simpler code, we also do not silently
break if there's a script around passing multiple files to
check.
Factor out the code from md5sum and sha1sum into a util function.
Use FILE * instead of a file descriptor. This will make it a bit
easier/more consistent when we implement support for the -c option.
