Fix fire auto-swapweaponing to polearm

When you have a polearm as secondary weapon, have a fireassist on,
and press 'f' to fire, the code tries to swapweapon to the polearm.
This failed badly and got stuck in a loop if you were also wearing
a shield - as polearms are two-handed and shield prevents wielding
those.

Add a new "command failed" result, and clear the command queue
in that case. Also make swapweapon and wield actually return
the ECMD flags back to the rhack loop.

src/cmd.c

@@ -3976,9 +3976,9 @@ rhack(char *cmd)
                 prefix_seen = 0;
                 was_m_prefix = FALSE;
             }
-            if ((res & ECMD_CANCEL)) {
+            if ((res & (ECMD_CANCEL|ECMD_FAIL))) {
                 /* command was canceled by user, maybe they declined to
-                   pick an object to act on. */
+                   pick an object to act on, or command failed to finish */
                 reset_cmd_vars(TRUE);
                 prefix_seen = 0;
                 cmdq_ec = NULL;