pgsql: Allow multi-character source strings in contrib/unaccent.

Allow multi-character source strings in contrib/unaccent.

This could be useful in languages where diacritic signs are represented as
separate characters; more generally it supports using unaccent dictionaries
for substring substitutions beyond narrowly conceived "diacritic removal".
In any case, since the rule-file parser doesn't complain about
multi-character source strings, it behooves us to do something unsurprising
with them.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/1b2488731cc2c87cc9a4cb8d654e4d9981fdf9ac

Modified Files
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contrib/unaccent/unaccent.c | 91 ++++++++++++++++++++++++++++---------------
doc/src/sgml/unaccent.sgml | 8 ++++
2 files changed, 67 insertions(+), 32 deletions(-)