pgsql: Improve test coverage of network address functions

Improve test coverage of network address functions

The following functions were not covered by any tests:
- abbrev(inet)
- set_masklen(cidr)
- set_masklen(inet)
- netmask(inet)
- hostmask(inet)

While on it, this improves the output of some of the existing queries in
the test inet to use better aliases.

Author: Aleksander Alekseev
Reviewed-by: Jacob Champion, Keisuke Kuroda, Tom Lane
Discussion: /messages/by-id/CAJ7c6TOyZ9bGNrDK6Z3Q0gr9ow8ZpOm+=+01mpE0dsdH4C+u9A@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/4f071349c0c28543fa8b84a5ce0b2f619c089ace

Modified Files
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src/test/regress/expected/inet.out | 163 +++++++++++++++++++++++++++----------
src/test/regress/sql/inet.sql | 21 +++--
2 files changed, 135 insertions(+), 49 deletions(-)