pgsql: Offer pnstrdup to frontend code

Offer pnstrdup to frontend code

We already had it on the backend.  Frontend can also use it now.

Discussion: https://postgr.es/m/20191204144021.GA17976@alvherre.pgsql

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/0b9466fce2cf4f8c32b3a9170ca272829aa11e66

Modified Files
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src/bin/pg_waldump/pg_waldump.c          |  3 +--
src/bin/psql/prompt.c                    | 16 +++++-----------
src/bin/scripts/common.c                 |  3 +--
src/common/fe_memutils.c                 | 27 +++++++++++++++++++++++++++
src/include/common/fe_memutils.h         |  1 +
src/interfaces/ecpg/compatlib/informix.c | 21 +--------------------
6 files changed, 36 insertions(+), 35 deletions(-)