tipc: Split up unified structure of network-related variables

Converts the fields of the global "tipc_net" structure into individual
variables.  Since the struct was never referenced as a complete unit,
its existence was pointless.  This will facilitate upcoming changes to
TIPC's node table and simpify upcoming relocation of the variables so
they are only visible to the files that actually use them.

This change is essentially cosmetic in nature, and doesn't affect the
operation of TIPC.

Signed-off-by: Allan Stephens <allan.stephens@windriver.com>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>