Make the directory where the pg_upgrade binary resides the default for
new bindir, as running the pg_upgrade binary from where the new
cluster is installed is a very common scenario.  Setting this as the
defauly bindir for the new cluster will remove the need to provide it
explicitly via -B in many cases.

To support directories being missing from option parsing, extend the
directory check with a missingOk mode where the path must be filled at
a later point before being used.  Also move the exec_path check to
earlier in setup to make sure we know the new cluster bindir when we
scan for required executables.

This removes the exec_path from the OSInfo struct as it is not used
anywhere.

Author: Daniel Gustafsson <daniel@yesql.se>
Reviewed-by: Peter Eisentraut <peter.eisentraut@2ndquadrant.com>
Discussion: https://www.postgresql.org/message-id/flat/9328.1552952117@sss.pgh.pa.us