Add TX/RX offload capabilities to shmif(4). They are emulated in
software by ether_sw_offload_[tr]x().

For rump kernels, if_capabilities for shmemif(4) can be specified
by environmental variable RUMP_SHMIF_CAPENABLE:

setenv RUMP_SHMIF_CAPENABLE 0x7ff80 (all offload)
setenv RUMP_SHMIF_CAPENABLE 0x6aa80 (all TX)
setenv RUMP_SHMIF_CAPENABLE 0x15500 (all RX)

part of PR kern/53562
OK msaitoh

(rin)