pgsql: Show a sensible value in pg_settings.unit for GUC_UNIT_XSEGS var

Show a sensible value in pg_settings.unit for GUC_UNIT_XSEGS variables.

Commit 88e982302 invented GUC_UNIT_XSEGS for min_wal_size and max_wal_size,
but neglected to make it display sensibly in pg_settings.unit (by adding a
case to the switch in GetConfigOptionByNum).  Fix that, and adjust said
switch to throw a run-time error the next time somebody forgets.

In passing, avoid using a static buffer for the output string --- the rest
of this function pstrdup's from a local buffer, and I see no very good
reason why the units code should do it differently and less safely.

Per report from Otar Shavadze.  Back-patch to 9.5 where the new unit type
was added.

Report: <CAG-jOyA=iNFhN+yB4vfvqh688B7Tr5SArbYcFUAjZi=0Exp-Lg@mail.gmail.com>

Branch
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REL9_6_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/993d94c59038fb80068a2b7780a1062dcaa2bb4f

Modified Files
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src/backend/utils/misc/guc.c | 20 ++++++++++++++------
src/include/utils/guc.h      |  4 ++--
2 files changed, 16 insertions(+), 8 deletions(-)