REORG: include: split common/regex.h into haproxy/regex{,-t}.h

Regex are essentially included for myregex_t but it turns out that
several of the C files didn't include it directly, relying on the
one included by their own .h. This has been cleanly addressed so
that only the type is included by H files which need it, and adding
the missing includes for the other ones.
diff --git a/src/http_htx.c b/src/http_htx.c
index 38a91cc..af8758c 100644
--- a/src/http_htx.c
+++ b/src/http_htx.c
@@ -15,6 +15,7 @@
 #include <unistd.h>
 
 #include <haproxy/api.h>
+#include <haproxy/regex.h>
 #include <types/global.h>
 
 #include <common/cfgparse.h>