pg_upgrade:  prefix Unix shell script name output with "./"

This more clearly suggests the current directory.  While this also works
on Windows, it might be confusing.

Report by Christoph Berg
Message improvements
pg_recvlogical: Improve --help output

List the actions first, as they are the most important options.  Group
the other options more sensibly, consistent with the man page.  Correct
a few typographical errors, clarify some things.

Also update the pg_receivexlog --help output to make it a bit more
consistent with that of pg_recvlogical.