Re: psql - add ability to test whether a variable exists

The following review has been posted through the commitfest application:
make installcheck-world:  not tested
Implements feature:       tested, failed
Spec compliant:           not tested
Documentation:            tested, failed

The patch applies cleanly and compiles + installs fine (although am unable to do installcheck-world on my Cygwin setup).
This is how the patch works on my setup.

$ /opt/postgres/reviewpatch/bin/psql -U postgres -h localhost
psql (11devel, server 9.6.1)
Type "help" for help.

postgres=# \set i 1
postgres=# \if :{?i}
postgres=# \echo 'testing'
testing
postgres=# \endif
postgres=# \if :{?id}
postgres@# \echo 'testing'
\echo command ignored; use \endif or Ctrl-C to exit current \if block
postgres@# \endif
postgres=#