MEDIUM: sample: handle comma-delimited converter list

We now support having a comma-delimited converter list, which can start
right after the fetch keyword. The immediate benefit is that it allows
to use converters in log-format expressions, for example :

   set-header source-net %[src,ipmask(24)]

The parser is also slightly improved and should be more resilient against
configuration errors. Also, optional arguments in converters were mistakenly
not allowed till now, so this was fixed.
MINOR: sample: fix sample_process handling of unstable data

sample_process() used to return NULL on changing data, regardless of the
SMP_OPT_FINAL flag. Let's change this so that it is now possible to
include such data in logs or HTTP headers. Also, one unconvenient
thing was that it used to always set the sample flags to zero, making
it incompatible with ACLs which may need to call it multiple times. Only
do this for locally-allocated samples.
CLEANUP: acl: move the 3 remaining sample fetches to samples.c

There is no more reason for having "always_true", "always_false" and "env"
in acl.c while they're the most basic sample fetch keywords, so let's move
them to sample.c where it's easier to find them.
MINOR: sample: add a new "date" fetch to return the current date

Returns the current date as the epoch (number of seconds since 01/01/1970).
If an offset value is specified, then it is a number of seconds that is added
to the current date before returning the value. This is particularly useful
to compute relative dates, as both positive and negative offsets are allowed.
MINOR: samples: add the http_date([<offset>]) sample converter.

Converts an integer supposed to contain a date since epoch to
a string representing this date in a format suitable for use
in HTTP header fields. If an offset value is specified, then
it is a number of seconds that is added to the date before the
conversion is operated. This is particularly useful to emit
Date header fields, Expires values in responses when combined
with a positive offset, or Last-Modified values when the
offset is negative.