pgsql: Tighten handling of "ago" in interval values

Tighten handling of "ago" in interval values

This commit Restrict the unit "ago" to only appear at the end of the
interval.  According to the documentation, a direction can only be
defined at the end of an interval, but it was possible to define it in
the middle of the string or define it multiple times.

In spirit, this is similar to the error handling improvements done in
5b3c5953553b or bcc704b524904.

Author: Joseph Koshakow
Reviewed-by: Jacob Champion, Gurjeet Singh, Reid Thompson
Discussion: https://postgr.es/m/CAAvxfHd-yNO+XYnUxL=GaNZ1n+eE0V-oE0+-cC1jdjdU0KS3iw@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/165d581f146b09543b832513ee00fead132ba6b1

Modified Files
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src/backend/utils/adt/datetime.c       | 7 +++++++
src/test/regress/expected/interval.out | 9 +++++++++
src/test/regress/sql/interval.sql      | 4 ++++
3 files changed, 20 insertions(+)