pgsql: Support non-ASCII letters in psql variable names.

Support non-ASCII letters in psql variable names.

As in the backend, the implementation actually accepts any non-ASCII
character, but we only document that you can use letters.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/e86fdb0ab224eaa73d907ab16a2dd0e0058699e0

Modified Files
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doc/src/sgml/ref/psql-ref.sgml | 25 ++++++-----
src/bin/psql/command.c | 6 +-
src/bin/psql/psqlscan.l | 87 ++++++++++++++++++++++++++++++++--------
src/bin/psql/variables.c | 34 +++++++++++++++-
src/bin/psql/variables.h | 4 --
5 files changed, 119 insertions(+), 37 deletions(-)