MEDIUM: config: allow to manipulate environment variables in the global section

MEDIUM: config: allow to manipulate environment variables in the global section

With new init systems such as systemd, environment variables became a
real mess because they're only considered on startup but not on reload
since the init script's variables cannot be passed to the process that
is signaled to reload.

This commit introduces an alternative method consisting in making it
possible to modify the environment from the global section with directives
like "setenv", "unsetenv", "presetenv" and "resetenv".

Since haproxy supports loading multiple config files, it now becomes
possible to put the host-dependant variables in one file and to
distribute the rest of the configuration to all nodes, without having
to deal with the init system's deficiencies.

Environment changes take effect immediately when the directives are
processed, so it's possible to do perform the same operations as are
usually performed in regular service config files.