Documentation/pktgen: Clearify how-to use pktgen samples

o Change process name in ps output: looks like, these days the process
is named kpktgend_<cpu>, rather than pktgen/<cpu>.
o Use pg_ctrl for start/stop as it can work well with pgset without
changes to $(PGDEV) variable.
o Clarify a bit needed $(PGDEV) definition for sample scripts and that
one needs to `source functions.sh`.
o Document how-to unset a behaviour flag, note about history expansion.
o Fix pgset spi parameter value.

Cc: Jonathan Corbet <[e-mail]>
Cc: [e-mail]
Signed-off-by: Dmitry Safonov <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

Documentation/networking/pktgen.txt

···
 On a dual CPU:
 
 ps aux | grep pkt
-root       129  0.3  0.0     0    0 ?        SW    2003 523:20 [pktgen/0]
-root       130  0.3  0.0     0    0 ?        SW    2003 509:50 [pktgen/1]
+root       129  0.3  0.0     0    0 ?        SW    2003 523:20 [kpktgend_0]
+root       130  0.3  0.0     0    0 ?        SW    2003 509:50 [kpktgend_1]
 
 
 For monitoring and control pktgen creates:
···
 ===================
 This is done via the /proc interface, and most easily done via pgset
 as defined in the sample scripts.
+You need to specify PGDEV environment variable to use functions from sample
+scripts, i.e.:
+export PGDEV=/proc/net/pktgen/eth4@0
+source samples/pktgen/functions.sh
 
 Examples:
+
+ pg_ctrl start           starts injection.
+ pg_ctrl stop            aborts injection. Also, ^C aborts generator.
 
  pgset "clone_skb 1"     sets the number of copies of the same packet
  pgset "clone_skb 0"     use single SKB for all transmits
···
                               IPSEC # IPsec encapsulation (needs CONFIG_XFRM)
                               NODE_ALLOC # node specific memory allocation
                               NO_TIMESTAMP # disable timestamping
+ pgset 'flag ![name]'    Clear a flag to determine behaviour.
+                         Note that you might need to use single quote in
+                         interactive mode, so that your shell wouldn't expand
+                         the specified flag as a history command.
 
- pgset spi SPI_VALUE     Set specific SA used to transform packet.
+ pgset "spi [SPI_VALUE]" Set specific SA used to transform packet.
 
  pgset "udp_src_min 9"   set UDP source port min, If < udp_src_max, then
                          cycle through the port range.
···
 
  pgset "tos XX"           set former IPv4 TOS field (e.g. "tos 28" for AF11 no ECN, default 00)
  pgset "traffic_class XX" set former IPv6 TRAFFIC CLASS (e.g. "traffic_class B8" for EF no ECN, default 00)
-
- pgset stop    	          aborts injection. Also, ^C aborts generator.
 
  pgset "rate 300M"        set rate to 300 Mb/s
  pgset "ratep 1000000"    set rate to 1Mpps