pgsql: Allow empty replacement strings in contrib/unaccent.

Allow empty replacement strings in contrib/unaccent.

This is useful in languages where diacritic signs are represented as
separate characters; it's also one step towards letting unaccent be used
for arbitrary substring substitutions.

In passing, improve the user documentation for unaccent, which was sadly
vague about some important details.

Mohammad Alhashash, reviewed by Abhijit Menon-Sen

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/97c40ce61465582b96944e41ed6ec06c2016b95c

Modified Files
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contrib/unaccent/unaccent.c | 29 +++++++++++++++++++++++------
doc/src/sgml/unaccent.sgml | 36 +++++++++++++++++++++++++++++++-----
2 files changed, 54 insertions(+), 11 deletions(-)