ntpd: add support for MD5/SHA1 message authentication

ntpd: add support for MD5/SHA1 message authentication

Add support for MD5 message authentication as described in RFC 5905.
This patch also supports SHA1 authentication.

The key file format is the same file format as used by ntpd.
The configuration file format follows standard Unix conventions
(# comments) with lines consist of the following fields separated by whitespace:
<key identifier, [1,65535]> <SHA1|MD5> <an ASCII string of up to 20 characters|an octet string [a-zA-F0-9] of up to 40 characters>.

https://www.ietf.org/rfc/rfc5905.txt

function                                             old     new   delta
ntp_init                                             473     987    +514
hash                                                   -     125    +125
recv_and_process_peer_pkt                            889     961     +72
packed_usage                                       33066   33130     +64
ntpd_main                                           1226    1277     +51
find_key_entry                                         -      29     +29
add_peers                                            195     207     +12
recv_and_process_client_pkt                          509     514      +5
------------------------------------------------------------------------------
(add/remove: 2/0 grow/shrink: 6/0 up/down: 872/0)             Total: 872 bytes

Signed-off-by: Brandon P. Enochs <enochs.brandon@gmail.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>