pgsql: Add pg_string_endswith as the start of a string helper library i

Add pg_string_endswith as the start of a string helper library in src/common.

Backpatch to 9.3 where src/common was introduce, because a bugfix that
needs to be backpatched, requires the function. Earlier branches will
have to duplicate the code.

Branch
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REL9_3_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/bb2e2ce6e2ea4835ed99593508d0909af0e402d6

Modified Files
--------------
src/common/Makefile         |    2 +-
src/common/string.c         |   43 +++++++++++++++++++++++++++++++++++++++++++
src/include/common/string.h |   15 +++++++++++++++
src/tools/msvc/Mkvcbuild.pm |    2 +-
4 files changed, 60 insertions(+), 2 deletions(-)