Fix multibyte issue in ltree_strncasecmp().

Fix multibyte issue in ltree_strncasecmp().

Previously, the API for ltree_strncasecmp() took two inputs but only
one length (that of the smaller input). It truncated the larger input
to that length, but that could break a multibyte sequence.

Change the API to be a check for prefix equality (possibly
case-insensitive) instead, which is all that's needed by the
callers. Also, provide the lengths of both inputs.

Reviewed-by: Chao Li <[email protected]>
Reviewed-by: Peter Eisentraut <[email protected]>
Discussion: https://postgr.es/m/5f65b85740197ba6249ea507cddf609f84a6188b.camel%40j-davis.com
Backpatch-through: 14