pgsql: 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 <li(dot)evan(dot)chao(at)gmail(dot)com>
Reviewed-by: Peter Eisentraut <peter(at)eisentraut(dot)org>
Discussion: https://postgr.es/m/5f65b85740197ba6249ea507cddf609f84a6188b.camel%40j-davis.com
Backpatch-through: 14

Branch
------
REL_18_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/f79e239e0bc6e4d5fe91e1a0e573ecf0715d6c8c

Modified Files
--------------
contrib/ltree/lquery_op.c | 40 ++++++++++++++++++++++++++++------------
contrib/ltree/ltree.h | 7 +++++--
contrib/ltree/ltxtquery_op.c | 8 ++++----
3 files changed, 37 insertions(+), 18 deletions(-)