DOC: update coding-style to reference checkpatch.pl

DOC: update coding-style to reference checkpatch.pl

Running the Linux kernel's checkpatch.pl is actually quite efficient
at spotting style issues and even sometimes bugs. The doc now suggests
how to use it to avoid the warnings that are specific to Linux's stricter
rules.

It properly reports errors like the following ones that were found on
real submissions so it should improve the situation for everyone :

ERROR: "foo * bar" should be "foo *bar"
+static char * tcpcheck_get_step_comment(struct check *, int);

ERROR: do not use assignment in if condition
+                       if ((comment = tcpcheck_get_step_comment(check, step)))

WARNING: trailing semicolon indicates no statements, indent implies otherwise
+                       if (elem->data && elem->free);
+                               elem->free(elem->data);

ERROR: do not initialise statics to 0 or NULL
+static struct lru64_head *ssl_ctx_lru_tree = NULL;

ERROR: space required after that ',' (ctx:VxV)
+           !X509_gmtime_adj(X509_get_notAfter(newcrt),(long)60*60*24*365))
                                                      ^
WARNING: space prohibited between function name and open parenthesis '('
+       else if (EVP_PKEY_type (capkey->type) == EVP_PKEY_RSA)

ERROR: trailing statements should be on next line
+       if (cacert) X509_free(cacert);

ERROR: space prohibited after that open parenthesis '('
+                                       !(         (srv_op_state == SRV_ST_STOPPED)