REORG: cfgparse: move server keyword parsing to server.c

The cfgparse.c file becomes huge, and a large part of it comes from the
server keyword parser. Since the configuration is a bit more modular now,
move this parser to server.c.

This patch also moves the check of the "server" keyword earlier in the
supported keywords list, resulting in a slightly faster config parsing
for configs with large numbers of servers (about 10%).

No functional change was made, only the code was moved.
diff --git a/include/proto/server.h b/include/proto/server.h
index 750d8d5..f030962 100644
--- a/include/proto/server.h
+++ b/include/proto/server.h
@@ -37,6 +37,7 @@
 int srv_downtime(const struct server *s);
 int srv_lastsession(const struct server *s);
 int srv_getinter(const struct check *check);
+int parse_server(const char *file, int linenum, char **args, struct proxy *curproxy, struct proxy *defproxy);
 
 /* increase the number of cumulated connections on the designated server */
 static void inline srv_inc_sess_ctr(struct server *s)