pgsql: Support Linux's oom_score_adj API as well as the older oom_adj A

Support Linux's oom_score_adj API as well as the older oom_adj API.

The simplest way to handle this is just to copy-and-paste the relevant
code block in fork_process.c, so that's what I did. (It's possible that
something more complicated would be useful to packagers who want to work
with either the old or the new API; but at this point the number of such
people is rapidly approaching zero, so let's just get the minimal thing
done.) Update relevant documentation as well.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/93f4d7f806613d28842f956a84f31cc41d825503

Modified Files
--------------
contrib/start-scripts/linux | 14 ++++++++---
doc/src/sgml/runtime.sgml | 23 +++++++++++++------
src/backend/postmaster/fork_process.c | 38 ++++++++++++++++++++++++++++----
3 files changed, 59 insertions(+), 16 deletions(-)