MEDIUM: acl: remove 15 additional useless ACLs that are equivalent to their fetches

The following 15 ACLs were missed from previous review, and are not needed
either.

hdr_cnt, hdr_ip, hdr_val, rep_ssl_hello_type, req_len, req_ssl_hello_type,
scook_cnt, scook_val, shdr_cnt, shdr_ip, shdr_val, url_ip, url_port,
urlp_val, req_proto_http.
DOC: major reorg of ACL + sample fetch

The split between ACL and sample fetch was a terrible mess in the doc,
as it caused all entries to be duplicated with most of them not easy to
find, some missing and some wrong.

The new approach consists in describing the sample fetch methods and
indicating the ACLs that are derived from these fetches. The doc is
much smaller (1500 lines added, 2200 removed, net gain = 700 lines)
and much clearer.

The description of the ACL mechanics was revamped to take account of
the latest evolutions and clearly describe the compatibility between
types of fetches and ACL patterns.

The deprecated keywords have been marked as such, though they still
appear in the examples given for various other keywords.
CLEANUP: http: remove the bogus urlp_ip ACL match

This one is wrong, never matches and cannot work. It was brought by a blind
copy-paste from the url_* version in 1.5-dev9, but there is no underlying
fetch returning an IP type for this.
MINOR: acl: add the new "env()" fetch method to retrieve an environment variable

This is useful in order to take different actions across restarts without
touching the configuration (eg: soft-stop), or to pass some information
such as the local host name to the next hop.
BUG/MINOR: acl: correctly consider boolean fetches when doing casts

Commit 5adeda1 (acl: add option -m to change the pattern matching method)
was not completely correct with regards to boolean fetches. It only used
the sample type to determine if the test had to be performed as a boolean
instead of relying on the match function. Due to this, a test such as the
following would not correctly match as the pattern would be ignored :

    acl srv_down srv_is_up(s2) -m int 0

No backport is needed as this was merged first in 1.5-dev18.