pgsql: Do not DROP default roles in pg_dumpall -c

Do not DROP default roles in pg_dumpall -c

When pulling the list of roles to drop, exclude roles whose names
begin with "pg_" (as we do when we are dumping the roles out to
recreate them).

Also add regression tests to cover pg_dumpall -c and this specific
issue.

Noticed by Rushabh Lathia.  Patch by me.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/018eb027f181234be7a580e9502c40ac5ad04f77

Modified Files
--------------
src/bin/pg_dump/pg_dumpall.c     |  8 +++-
src/bin/pg_dump/t/002_pg_dump.pl | 87 +++++++++++++++++++++++++++++++++++++++-
2 files changed, 93 insertions(+), 2 deletions(-)