netfilter: Implement RFC 1123 for FTP conntrack

 The FTP conntrack code currently only accepts the following format for
 the 227 response for PASV:
 227 Entering Passive Mode (148,100,81,40,31,161).

 It doesn't accept the following format from an obscure server:
 227 Data transfer will passively listen to 67,218,99,134,50,144

 From RFC 1123:
 The format of the 227 reply to a PASV command is not
 well standardized.  In particular, an FTP client cannot
 assume that the parentheses shown on page 40 of RFC-959
 will be present (and in fact, Figure 3 on page 43 omits
 them).  Therefore, a User-FTP program that interprets
 the PASV reply must scan the reply for the first digit
 of the host and port numbers.

 This patch adds support for the RFC 1123 clarification by:
 - Allowing a search filter to specify NUL as the terminator so that
   try_number will return successfully if the array of numbers has been
   filled when an unexpected character is encountered.
 - Using space as the separator for the 227 reply ...