build: allow configure and build in a separate directory

Some .h files in lib/ are autogenerated. The search path should
include the build directory and the source directory. They usually
match but sometimes, they may be different. For example:

 $ mkdir build
 $ cd build
 $ ../configure
 $ make
build: only define HAVE_SNMP

NetSNMP is the only SNMP implementation for Quagga. We don't need two
different symbols.
build: use net-snmp-config to configure NetSNMP

The correct method to link to NetSNMP is to use net-snmp-config (which
is like pkg-config). Explicit link to libcrypto is also dropped
(NetSNMP libs are linked to libcrypto, no need to link Quagga to
it). Moreover, @SNMP_INCLUDES@ is dropped because useless. Due to a
bug in configure.ac, it was properly populated.
smux: isolate SMUX implementation from SNMP implementation

lib/snmp.c gets OID related helper functions that can be used with
another SNMP interface. smux.h is cleaned of SMUX specific bits to
only expose functions that may be used by an alternative
implementation. We also do not redefine functions already present in
NetSNMP. Just use the appropriate headers.
agentx: add AgentX support to Quagga.

--enable-snmp will enable AgentX support in Quagga. SMUX is still here
and can be enabled with --enable-snmp=smux. AgentX support can be
enabled with "agentx" in configuration file. As for SMUX, this command
is not understood by vtysh. It can be disabled with "no agentx",
though there is no real use of this since this command cannot be used
with vtysh.

If "agentx" and "no agentx" command were added to vtysh, it would not
be possible to disable agentx support after enabling it because
NetSNMP does not expose the appropriate methods for this.

The internals of AgentX are hidden by NetSNMP. Therefore, we don't
have a file descriptor to add to the threading system. We do not have
the timers to set either. Therefore, the event loop is modified to
make use of snmp_select_info() from NetSNMP.

Traps are not supported yet.
smux: remove `tick` argument from smux_trap()

smux_trap() contains an argument whose use appears to be to set
sysUpTime.0/timestamp field in SNMP trap. However, this value is not
used in smux_trap(). Moreover, it is expected that this field is the
value of sysUpTime.0 when the trap was sent and not any other time
related to the trap. To avoid any confusion, we remove this field from
the signature of the function.
smux: drop findVar element from trap object struct

This element was not unused.
agentx: handle SNMP traps

smux_trap() signature has been changed to provide appropriate level
information to send SNMPv2 notifications. This includes the addition
of the enterprise OID to use (from which is derived the SNMP trap OID)
and the MIB registry to locate the appropriate function for variable
bindings provided by the trap.

The SMUX implementation has been updated but ignore the provided
enterprise OID. Instead, it still uses the SMUX peer OID to keep
compatibility with previous versions of Quagga. The SMUX
implementation also ignores the provided MIB registry since it uses
smux_get() function to grab the appropriate values. This is not
possible with the AgentX implementation since there is no such
function provided by NetSNMP.
agentx: add appropriate documentation
snmp: let handlers accept OID from a lesser prefix

Most table handlers do not expect to be given an OID whose prefix is
outside what they can handle. This is not a problem with the SMUX
implementation since it always correct the OID such that the prefix
matches. However, this is not the case for the AgentX
implementation. A new function, smux_header_table() is used to do this
normalization.
bgpd: upgrade SNMP support according to RFC 4273