vauth: Corrected a number of typos in comments

lib/vauth/digest.c

lib/vauth/digest_sspi.c

lib/vauth/krb5_gssapi.c

lib/vauth/krb5_sspi.c

lib/vauth/spnego_gssapi.c

lib/vauth/spnego_sspi.c

lib/vauth/vauth.c