crypt: Remove invalid end of page test badsalttest

The input argument passes an invalid string without a NUL terminator
on crypt settings inputs, which might lead to invalid OOB on strncmp.

Implementations only assume there is a NUL terminator if the string is
shorter than the specified size, so strings don't need to always be NUL
terminated (stratcliff.c has tests for this).

Also adapt the code to use libsupport.

Checked on arm-linux-gnuabihf.