BUG/MEDIUM: tcp: process could theorically crash on lack of source ports

When connect() fails with EAGAIN or EADDRINUSE, an error message is
sent to logs and uses srv->id to indicate the server name (this is
very old code). Since version 1.4, it is possible to have srv == NULL,
so the message could cause a crash when connect() returns EAGAIN or
EADDRINUSE. However in practice this does not happen because on lack
of source ports, EADDRNOTAVAIL is returned instead, so this code is
never called.

This fix consists in not displaying the server name anymore, and in
adding the test for EADDRNOTAVAIL.

Also, the log level was lowered from LOG_EMERG to LOG_ERR in order
not to spam all consoles when source ports are missing for a given
target.

This fix should be backported to 1.4.
MINOR: cfgparse: mention "interface" in the list of allowed "source" options

"interface" was only mentionned for the proxy source address but not
for the server's.
MEDIUM: connection: introduce "struct conn_src" for servers and proxies

Both servers and proxies share a common set of parameters for outgoing
connections, and since they're not stored in a similar structure, a lot
of code is duplicated in the connection setup, which is one sensible
area.

Let's first define a common struct for these settings and make use of it.
Next patches will de-duplicate code.

This change also fixes a build breakage that happens when USE_LINUX_TPROXY
is not set but USE_CTTPROXY is set, which seem to be very unlikely
considering that the issue was introduced almost 2 years ago an never
reported.
CLEANUP: proto_tcp: use the same code to bind servers and backends

The tproxy and source binding code has now be factored out for
servers and backends. A nice effect is that the code now supports
having backends use source port ranges, though the config does not
support it yet. This change has reduced the executable by around
700 bytes.
CLEANUP: backend: use the same tproxy address selection code for servers and backends

This is just like previous commit, but for the backend this time. All this
code did not need to remain duplicated. These are 500 more bytes shaved off.
BUG/MEDIUM: stick-tables: conversions to strings were broken in dev13

Commit 07115412 (MEDIUM: stick-table: allocate the table key...) broke
conversion of samples to strings for stick tables, because if replaced
char buf[BUFSIZE] with char buf[0] and the string converters use sizeof
on this part. Note that sizeof was wrong as well but at least it used
to work.

Fix this by making use of the len parameter instead of sizeof.
MEDIUM: proto_tcp: add support for tracking L7 information

Until now it was only possible to use track-sc1/sc2 with "src" which
is the IPv4 source address. Now we can use track-sc1/sc2 with any fetch
as well as any transformation type. It works just like the "stick"
directive.

Samples are automatically converted to the correct types for the table.

Only "tcp-request content" rules may use L7 information, and such information
must already be present when the tracking is set up. For example it becomes
possible to track the IP address passed in the X-Forwarded-For header.

HTTP request processing now also considers tracking from backend rules
because we want to be able to update the counters even when the request
was already parsed and tracked.

Some more controls need to be performed (eg: samples do not distinguish
between L4 and L6).
MEDIUM: counters: add sc1_trackers/sc2_trackers

Returns the current amount of concurrent connections tracking the same
tracked counters. This number is automatically incremented when tracking
begins and decremented when tracking stops. It differs from sc1_conn_cur in
that it does not rely on any stored information but on the table's reference
count (the "use" value which is returned by "show table" on the CLI). This
may sometimes be more suited for layer7 tracking.
MINOR: http: add the "base32" pattern fetch function

This returns a 32-bit hash of the value returned by the "base"
fetch method above. This is useful to track per-URL activity on
high traffic sites without having to store all URLs. Instead a
shorter hash is stored, saving a lot of memory. The output type
is an unsigned integer.
MINOR: http: add the "base32+src" fetch method.

This returns the concatenation of the base32 fetch and the src fetch.
The resulting type is of type binary, with a size of 8 or 20 bytes
depending on the source address family. This can be used to track
per-IP, per-URL counters.