Split out from rdist/defs.h the client-only and server-only bits into

Split out from rdist/defs.h the client-only and server-only bits into
 rdist/client.h and rdistd/server.h
Only put #includes in .h files that are necessary for the .h to be used;
 all other #includes go in the .c files
Move all extern variable declarations to the .h files, renaming local
 variables to avoid shadowing
Replace me_type member of mntent_t with an "is NFS?" flag bit