Change handling of invalid JSON and XML request body

[creepyghost]

Description

• This MR changes the behaviour of modsecurity when handling invalid request JSON or XML.
• Modsecurity currently fails at parsing these request bodies when content type is specified
  and throws an HTTP 500 status. This behaviour isn't exactly compliant with the HTTP RFC.
• This MR proposes a standardized mechanism for handling body parser errors.
• The return codes are standardized in the common header file and a special return code
  BODY_PARSER_ERR_INVALID_BODY now indicates that the body parser failed.
• Modsecurity proceeds to evaluate the standard modsec-recommended rule 200002 which
  handles the REQBODY_ERROR target to return a block decision with HTTP 400.

Changes

modsecurity.h

• Standardize body parser return codes

msc_json.c, msc_xml.c

• Return special error code on body parsing failure

msc_reqbody.c, apache2_io.c, mod_security2.c

• Change body parsing error codes to refer to constants
  defined in header.