md5: extend capabilites

In 4849767cb16a4, we did a large refactor of the md5(1) source code.
One of them is that instead of reading data using read(2) syscall, we
are using binary stream input (fread(3)).

fread(3) requires additional Capsicum capabilities:
sha256 CAP operation requires CAP_FSTAT, descriptor holds CAP_READ
sha256 RET fstat -1 errno 93 Capabilities insufficient

Reviewed by: des
Differential Revision: https://reviews.freebsd.org/D41348