ctype: use non-locale-specific ctype.h

ctype: use non-locale-specific ctype.h

We also make these constant time, even though we're never distinguishing
between bits of a secret using them. From that perspective, though, this
is markedly better than the locale-specific table lookups in glibc, even
though base64 characters span two cache lines and valid private keys
must hit both.

Co-authored-by: Samuel Neves <sneves@dei.uc.pt>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Samuel Neves <sneves@dei.uc.pt>