MINOR: cfgparse-global: add more checks for "chroot" argument

MINOR: cfgparse-global: add more checks for "chroot" argument

If directory provided as a "chroot" keyword argument does not exist or
inaccessible, this is reported only at the latest initialization stage, when
haproxy tries to perform chroot. Sometimes it's not very convenient, as the
process is already bound to listen sockets.

This was done explicitly in order not to break the case, when haproxy is
launched with "-c" option in some specific environment, where it's not possible
to create or to modify chroot directory, provided in the configuration.

So, let's add more checks for "chroot" directory during the parsing
stage and let's show diagnostic warnings, if this directory has become
non-accesible or was deleted. Like this, users, who wants to catch errors
related to misconfigured chroot before starting the process, can launch haproxy
with -dW and -dD. zero-warning mode will stop the process with error, if any
warning was emitted during initialization stage.