Make the encoding argument uniformly guint rather than gint.

Make the encoding argument uniformly guint rather than gint.

Put the code that's currently common to the ENC_ASCII and ENC_UTF_8
cases in tvb_get_ephemeral_string_enc() into tvb_get_ephemeral_string(),
and call tvb_get_ephemeral_string() in those cases.  Skip the
tvb_ensure_bytes_exist() and ensure_contiguous() calls in the ENC_UTF_16
and ENC_UCS_2 cases, as they're unnecessary there.

Update the comment for tvb_get_ephemeral_string_enc().

Make tvb_get_ephemeral_stringz_enc() handle the encodings that
tvb_get_ephemeral_string_enc() does.

svn path=/trunk/; revision=42627