Fix a wrong warning from gcc on recent stable and current

Reported by:	peter@
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

Conflicts:
	libpkg/pkg.h.in
	libpkg/pkg_config.c
Unify the mirroring type into one single option:

MIRROR_TYPE which can take the folloing arguments (case insensitive)
http: http mirroring
srv: srv mirroing
anythingbutprevious: no mirroring

Conflicts:
	libpkg/pkg.h.in
	libpkg/pkg_config.c
Allow users to define the proxies via pkg.conf

pkg uses by default libfetch and thus inherit from the environnement variable
that libfetch supports.

However some users might want to be able to define them only for pkg, now they
can do it via pkg.conf

Conflicts:
	libpkg/pkg.h.in
	libpkg/pkg_config.c