Add --if-modified-since option

* src/init.c: Add to commands array.
* src/main.c: Add to cmdline_option. Add to help message.
* src/options.h: Add to options struct.

src/init.c

@@ -205,6 +205,7 @@ static const struct {
 #endif
   { "httpsproxy",       &opt.https_proxy,       cmd_string },
   { "httpuser",         &opt.http_user,         cmd_string },
+  { "if-modified-since", &opt.if_modified_since, cmd_boolean },
   { "ignorecase",       &opt.ignore_case,       cmd_boolean },
   { "ignorelength",     &opt.ignore_length,     cmd_boolean },
   { "ignoretags",       &opt.ignore_tags,       cmd_vector },
@@ -361,6 +362,7 @@ defaults (void)
     opt.no_proxy = sepstring (tmp);
   opt.prefer_family = prefer_none;
   opt.allow_cache = true;
+  opt.if_modified_since = true;
 
   opt.read_timeout = 900;
   opt.use_robots = true;

src/main.c

@@ -301,6 +301,7 @@ static struct cmdline_option option_data[] =
     { "strict-comments", 0, OPT_BOOLEAN, "strictcomments", -1 },
     { "timeout", 'T', OPT_VALUE, "timeout", -1 },
     { "timestamping", 'N', OPT_BOOLEAN, "timestamping", -1 },
+    { "if-modified-since", 0, OPT_BOOLEAN, "if-modified-since", -1 },
     { "tries", 't', OPT_VALUE, "tries", -1 },
     { "unlink", 0, OPT_BOOLEAN, "unlink", -1 },
     { "trust-server-names", 0, OPT_BOOLEAN, "trustservernames", -1 },
@@ -515,6 +516,9 @@ Download:\n"),
     N_("\
   -N,  --timestamping              don't re-retrieve files unless newer than\n\
                                      local\n"),
+    N_("\
+       --no-if-modified-since      don't use conditional if-modified-since get\n\
+                                     requests in timestamping mode\n"),
     N_("\
   --no-use-server-timestamps       don't set the local file's timestamp by\n\
                                      the one on the server\n"),

src/options.h

@@ -165,6 +165,7 @@ struct options
 #endif
 
   bool timestamping;            /* Whether to use time-stamping. */
+  bool if_modified_since;       /* Whether to use conditional get requests.  */
 
   bool backup_converted;        /* Do we save pre-converted files as *.orig? */
   int backups;                  /* Are numeric backups made? */