CLEANUP: regex: Create regex_comp function that compiles regex using compilation options

The current file "regex.h" define an abstraction for the regex. It
provides the same struct name and the same "regexec" function for the
3 regex types supported: standard libc, basic pcre and jit pcre.

The regex compilation function is not provided by this file. If the
developper wants to use regex, he must write regex compilation code
containing "#define *JIT*".

This patch provides a unique regex compilation function according to
the compilation options.

In addition, the "regex.h" file checks the presence of the "#define
PCRE_CONFIG_JIT" when "USE_PCRE_JIT" is enabled. If this flag is not
present, the pcre lib doesn't support JIT and "#error" is emitted.
BUG/MEDIUM: http: accept full buffers on smp_prefetch_http

Bertrand Jacquin reported a but when using tcp_request content rules
on large POST HTTP requests. The issue is that smp_prefetch_http()
first tries to validate an input buffer, but only if the buffer is
not full. This test is wrong since it must only be performed after
the parsing has failed, otherwise we don't accept POST requests which
fill the buffer as valid HTTP requests.

This bug is 1.5-specific, no backport needed.