Makes Curl no longer a mandatory depedency for ModSecurity core

As reported by Rainer Jung, Curl may not be mandatory to build
	ModSecurity core. This patch make it optional by:
	- Concentrate all downloads using curl on msc_remote_rules.c
	- Split Curl build definitions checks into: WITH_CURL, WITH_REMOTE_RULES
	and WITH_CRYPTO.
	  - WITH_CURL: Contains Culr headers and binaries during the build time.
	  - WITH_REMOTE_RULES: Currently enabled if Curl is present.
	  - WITH_CRYPTO: Set if apr tool was compiled with crypto support.
	- Renames msc_remote_grab_content to msc_remote_download_content

build/find_curl.m4

@@ -70,6 +70,7 @@ if test -n "${curl_path}"; then
 	if test "$curl_tlsv2_ver" -le "$curl_ver"; then
 	    CURL_CFLAGS="${CURL_CFLAGS} -DWITH_CURL_SSLVERSION_TLSv1_2"
 	fi
+	CURL_CFLAGS="${CURL_CFLAGS} -DWITH_CURL"
     else
         AC_MSG_RESULT([no, $CURL_VERSION])
         AC_MSG_NOTICE([NOTE: curl library may be too old])