Use pg_get_triggerdef in pg_dump

Add a variant of pg_get_triggerdef with a second argument "pretty" that
causes the output to be formatted in the way pg_dump used to do.  Use this
variant in pg_dump with server versions >= 8.5.

This insulates pg_dump from most future trigger feature additions, such as
the upcoming column triggers patch.

Author: Itagaki Takahiro <itagaki.takahiro@oss.ntt.co.jp>