Convert cvsignore to gitignore, and add .gitignore for build targets.

doc/src/sgml/.cvsignore

doc/src/sgml/.gitignore

src/.gitignore

src/backend/.gitignore

src/backend/bootstrap/.cvsignore

src/backend/bootstrap/.gitignore

src/backend/catalog/.cvsignore

src/backend/catalog/.gitignore

src/backend/parser/.cvsignore

src/backend/parser/.gitignore

src/backend/port/.gitignore

src/backend/snowball/.gitignore

src/backend/utils/.cvsignore

src/backend/utils/.gitignore

src/backend/utils/mb/conversion_procs/.gitignore

src/backend/utils/misc/.cvsignore

src/backend/utils/misc/.gitignore

src/bin/initdb/.gitignore

src/bin/pg_config/.gitignore

src/bin/pg_controldata/.gitignore