memfrob transforms (frobnicates) each byte of the data structure
at mem, which is length bytes long, by bitwise exclusive
oring it with binary 00101010. It does the transformation in place and
its return value is always mem.
memfrob a second time on the same data structure
returns it to its original state.
This is a good function for hiding information from someone who doesn't
want to see it or doesn't want to see it very much. To really prevent
people from retrieving the information, use stronger encryption such as
that described in See Cryptographic Functions.
Portability Note: This function is unique to the GNU C library.