[3207] Added callouts for packet receive and send to user check hooks lib

Initial checkin creates callout functions for packet receive and send
hooks for IPv4 and IPv6.  This sets thes stage for outputting user specific
options in outbound packet.
[3207] Replaced selected subnet with lease address in user check output

User check hook library's outcome output is now generated in pkt_send
callout allowing the output of lease address (or prefix) in place
of selected subnet.
[3207] Link fix in user_chk
[3207] Added bootfile option output to user check hook library

Added ability to add the boot file option from the user registry to
the outbound DHCP response to user check hook library.
[3207] Added support for TFTP_SERVER option to user_chech hook library

User check hook library now supports adding TFTP server option to DHCP
response packet options from values in user registry.
[3207] Added support for default users and delimited user id input

Vendor option default values are now taken from default users if the
client is not registered.  Registry input file user ids can now contain
colons as delimiters allowing cut-and-paste from outcome file.
[3207] Clean and doxygen commentary for user_check hook library.

Adding doxygen and cleaned up code for initial review submission.
Merge branch 'master' into trac3207

3207 was created quite some time ago, so master was
merged into it first.
[3207] Addressed review comments.

Addressed review comments which were largely minor. Limited use of
extern C linkage to only the callout functions themselves. Added
a dox page describing the library.  Added namespace user_chk.
[3207] Some minor corrections to user_chk library dox page.
[3207] A few more minor review changes.

Removed reponse packet type check from IPv6 packet_snd callout.
Corrected text in user chk doxygen page.
Merge branch 'trac3207'

   Completes initial implmementation of example hooks libary, user_chk.
[master] Added ChangeLog entry 721 for trac3207.