Quick'n'dirty implementation of pkg+foo:// URL scheme, for foo in
(http, https, ftp).

Some style improvements

Code in libpkg/fetch.c needs refactoring so we can cleanly implement
pkg+ssh://

issue #550
etags generates a file called TAGS by default.
Use new pkg+http:// URL scheme

issue #550
Add some documentation about URL schemes in PACKAGESITE settings.

Issue #550