Backpatch certain files through 9.0

Prominent binaries already had this metadata.  A handful of minor
binaries, such as pg_regress.exe, still lack it; efforts to eliminate
such exceptions are welcome.

Michael Paquier, reviewed by MauMau.

Report by Robert Haas

Update all files in head, and files COPYRIGHT and legal.sgml in all back
branches.

Fully update git head, and update back branches in ./COPYRIGHT and
legal.sgml files.

strength of database passwords, and create a sample implementation of
such a hook as a new contrib module "passwordcheck".

Laurenz Albe, reviewed by Takahiro Itagaki