Add shared subtree support, suggested by Vladimir Dronnikov. Also break out a

few new (unfinished) config options, which I intend to make hidden (but
enabled) when CONFIG_NITPICK is disabled.  Getting the .config infrastructure
to do that is non-obvious, it seems...

libbb/mtab_file.c

@@ -13,8 +13,5 @@
 
 /* Busybox mount uses either /proc/mounts or /etc/mtab to
  * get the list of currently mounted filesystems */
-#if defined(CONFIG_FEATURE_MTAB_SUPPORT)
-const char bb_path_mtab_file[] = "/etc/mtab";
-#else
-const char bb_path_mtab_file[] = "/proc/mounts";
-#endif
+char bb_path_mtab_file[] =	USE_FEATURE_MTAB_SUPPORT("/etc/mtab")
+							SKIP_FEATURE_MTAB_SUPPORT("/proc/mounts");