New HTTP_MIRRORS option default to off

This feature has been requested for PCBSD mirror handling.
How it works:

PACKAGESITE: http://host/path

pkgng will fetch http://host
if fetch works the result is read looking for URL: http://newhost/newpath
the fetching http://host/path/pkg
will result into fetching http://newhost/newpath/path/pkg

While here add a new build time option to chose the default mirroring system via
the DEFAULT_MIRROR_TYPE value can be:
0: none
1: srv
2: http

Add a CFLAG to be able to define a hardcoded default packagesite at build time:
DEFAULT_PACKAGESITE (default to NULL)

Add a CFLAG to be able to change the default package audit url at build time:
DEFAULT_PORTAUDIT_URL (default to: http://portaudit.FreeBSD.org/auditfile.tbz)

To finish, only lookup once for the mirrors and protect looking up via mutext to
keep the library thread safe