libpq:  have PQconnectdbParams() and PQpingParams accept "" as default

Previously, these functions treated "" optin values as defaults in some
ways, but not in others, like when comparing to .pgpass.  Also, add
documentation to clarify that now "" and NULL use defaults, like
PQsetdbLogin() has always done.

BACKWARD INCOMPATIBILITY

Patch by Adrian Vondendriesch, docs by me

Report by Jeff Janes
docs:  tablespaces cannot be accessed independently

Mention impossibility of moving tablespaces, backing them up
independently, or the inadvisability of placing them on temporary
file systems.

Patch by Craig Ringer, adjustments by Ian Lawrence Warwick and me
doc: mention archive_command and recovery_command are exec'ed locally

Report by Craig Ringer
doc: CREATE DATABASE doesn't copy template database-level config params

Report by Alexey Bashtanov
Fix typo.

Etsuro Fujita
Fix another typo.

Etsuro Fujita