udhcpd: remove five more options which do not make sense or not supported

requestip, vendorclass, clientid are client-side variables,
          they do not make sense as udhcpd opts
dhcptype  is the packet type (not interesting, it's always 5)
userclass needs parser support in order to work

function                                             old     new   delta
dhcp_options                                          68      66      -2
read_opt                                             865     859      -6
dhcp_option_strings                                  253     203     -50
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 0/3 up/down: 0/-58)             Total: -58 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

examples/udhcp/udhcpd.conf

@@ -63,27 +63,35 @@ option	domain	local
 option	lease	864000		# 10 days of seconds
 
 # Currently supported options (for more info, see options.c):
-#opt subnet
-#opt timezone
-#opt router
-#opt timesrv
-#opt namesrv - obsolete, disabled
-#opt dns
-#opt logsrv
-#opt cookiesrv - rarely (never?) used, disabled
-#opt lprsrv
-#opt bootsize
-#opt domain
-#opt swapsrv
-#opt rootpath
-#opt ipttl
-#opt mtu
-#opt broadcast
-#opt wins
-#opt lease
-#opt ntpsrv
-#opt tftp
-#opt bootfile
+#opt lease      NUM
+#opt subnet     IP
+#opt broadcast  IP
+#opt router     IP_LIST
+#opt ipttl      NUM
+#opt mtu        NUM
+#opt hostname   STRING      - client's hostname
+#opt domain     STRING      - client's domain name
+#opt search     STRING_LIST - search domains
+#opt nisdomain  STRING
+#opt timezone   NUM
+#opt tftp       STRING - TFTP server name
+#opt bootfile   STRING - file (kernel image) to load for booting
+#opt bootsize   NUM    - size of that file
+#opt rootpath   STRING - (NFS) path to mount as root fs
+#opt wpad       STRING
+#opt serverid   IP     - by default, server's IP
+#opt message    STRING - error message (udhcpd sends it on success too)
+# Options specifying server(s)
+#opt dns        IP_LIST
+#opt wins       IP_LIST
+#opt nissrv     IP_LIST
+#opt namesrv    IP_LIST - obsolete, disabled
+#opt logsrv     IP_LIST
+#opt cookiesrv  IP_LIST - rarely (never?) used, disabled
+#opt lprsrv     IP_LIST
+#opt swapsrv    IP
+#opt timesrv    IP_LIST
+#opt ntpsrv     IP_LIST
 
 # Static leases map
 #static_lease 00:60:08:11:CE:4E 192.168.0.54