Default to normal salt instead of "ignore"

krb5_string_to_keysalts() treats an empty salt field as -1 ("ignore"),
rather than as the normal salttype.  Default to normal instead, so
that omitting a salttype works as expected.

ticket: 6042
Make salt defaulting work for keysalts

Make krb5_string_to_keysalts() default to only ":" as a key:salt
separator character.  Change most of its callers to pass NULL so they
get the default separators.

Adapted from a patch proposed by Jon Looney.

ticket: 884
Add test for salttype defaulting

ticket: 884