Incorrect use of "len" throughout http_parser.c

CHUNK_STRNCASECMP() is used in functions and parts of the code to find
and process specific HTTP headers and header tokens (fields). The macro
uses "len" argument that is set once in each function prior to the macro
use. As the macro is used and the FSM moves from one state to another,
the "p" pointer is moved forward as well, thus making the initial "len"
value no longer correct. Using the initial "len" value may therefore
lead to memory corruption.

- Fix CHUNK_STRNCASECMP() macro.
- Fix all places that use the initial "len" value.
- Fix several places where a "comma" pointer is incorrectly compared
  to an "lf" pointer (similar to 46f3eaeb)
- Correct a small number of comments for consistency and readability.