MASTER_SITES cleanup.

• Replace ${MASTER_SITE_FOO} with FOO.
• Merge MASTER_SITE_SUBDIR into MASTER_SITES when possible. (This means 99.9% of the time.)
• Remove occurrences of MASTER_SITE_LOCAL when no subdirectory was present and no hint of what it should be was present.
• Fix some logic.
• And generally, make things more simple and easy to understand.

While there, add magic values to the FESTIVAL, GENTOO, GIMP, GNUPG, QT and
SAMBA macros.

Also, replace some EXTRACT_SUFX occurences with USES=tar:*.

Checked by: make fetch-urlall-list
With hat: portmgr
Sponsored by: Absolight