Use c_char* instead of i8* for FFI to pam C wrapper
Use c_char* instead of i8* for FFI to pam C wrapper

Update the function definitions in the PAM FFI wrapper to declare string
parameters as c_char* instead of i8*. This fixes building on systems
with unsigned `char` types, like powerpc64le.

shawnanastasio/rudo,shawnanastasio/rudo,shawnanastasio/rudo