implement basic X-Forwarded-For header support

The commit introduces two major things:
1. The HTTP parser is now able to parse the X-Forwarded-For header and
   put it to the headers table, although t doesn't parse IP addresses
   or distinct Node IDs yet.
2. Client's IP address is added to X-Forwarded-For header of each
   forwarded HTTP request. If there is no such header - it is created,
   if it is already there, then the client's IP is appended to the list.

Also the commit contains fixes for the HTTP parser that used to crop
parsed HTTP headers and write them into each other's locations in the
headers table.

Related to: #5 - Load Balancing
revert HTTP parser fixes that are already merged to master
Fix IP address formatting issues noticed during the code review.
Fix output string temintation in tfw_str_to_cstr()
remove unnecessary buffer zero'ing in tfw_http_append_forwarded_for()

This should be a hot function, so we would like to remove all
unnecessary actions that may affect performance.
optimize tfw_str_to_cstr()  (noticed on code review, minor changes)
resolve conflicts with master (changes from fix-header-parsing branch)
resolve conflicts with master (changes from fix-header-parsing branch)
fix leading LWS and multiple comma parsing in X-Forwarded-For header

1. The problem with leading LWS is the RGEN_LWS() expects to see an
alpha-numeric character after LWS, but we can have '[' in an IPv6
address. Such requests were blocked before this commit.

2. Multiple commas in the header are suspicious. They are not allowed
anymore.
add fast path for 'X-Forwarded-For: ' parsing
Fix dash character comparisons in 'X-Forwarded-For' in HTTP parser
Merge branch 'master' into x-forwarded-for

Conflicts:
	tempesta_fw/http_parser.c
	tempesta_fw/t/Makefile
Merge branch 'x-forwarded-for'