pgsql: Support replacing MODULE_PATHNAME during extension script file e

Support replacing MODULE_PATHNAME during extension script file execution.

This avoids the need to find a way to make PGXS' .sql.in-to-.sql rule
insert the right thing.  We'll just deprecate use of that hack for
extensions.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/e693e97d754ed6812ea115170afeae4bf8797d3f

Modified Files
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doc/src/sgml/extend.sgml         |   31 +++++++++++++++++++++++--------
src/backend/commands/extension.c |   18 ++++++++++++++++++
2 files changed, 41 insertions(+), 8 deletions(-)