pg_basebackup: Add support for relocating tablespaces

Tablespaces can be relocated in plain backup mode by specifying one or
more -T olddir=newdir options.

Author: Steeve Lennmark <steevel@handeldsbanken.se>
Reviewed-by: Peter Eisentraut <peter_e@gmx.net>
configure.in: Use dnl in place of # where appropriate

The comment added by ed011d9754fd4b76eac0eaa8c057fcfc0c302a6a used #,
which means it gets copied into configure, but it doesn't make sense
there.  So use dnl, which gets dropped when creating configure.