if_awg: rename tx functions to match other drivers and free mbuf on m_collapse failure

• use awg_encap and awg_txeof names to match iflib and other network drivers.
• handle m_collapse failure similarly by freeing the mbuf rather than reenqueuing it where it will continue to fail.

Submitted by: Guy Yur <guyyur@gmail.com>
Differential Revision: https://reviews.freebsd.org/D13035