Adapt crypt code to Unikraft

Adapt crypt code to Unikraft

We tried to keep the changes of the original code to a minimum. For this
purpose, we used the C preprocessor in order to disable the 'hidden' musl
qualifier. We also had to rename the <crypt.h> header in
`musl-imported/src/include` to <_crypt.h> because, being a private header, we
could not add its directory at the top of the include paths list to include it
before the <crypt.h> header in `musl-imported/include` (the latter being
a public include path).

Signed-off-by: Costin Lupu <costin.lupu@cs.pub.ro>
Reviewed-by: Roxana Nicolescu <nicolescu.roxana1996@gmail.com>