MFC 297776,297777,297779: Add DDB commands to cxgbe(4).

MFC 297776,297777,297779: Add DDB commands to cxgbe(4).

297776:
Add a function to lookup a device_t object by name.

This just walks the global list of devices looking for one with the
requested name.  The one use case outside of devctl2's implementation
is for DDB commands that wish to lookup devices by name.

297777:
Add a 'show t4 tcb <nexus> <tid>' command to dump a TCB from DDB.

This allows the contents of a TCB to be extracted from a T4/T5 card in
DDB after a panic.

297779:
Add a 'show t4 devlog <nexus>' DDB command.

This command displays the adapter's firmware device log similar to the
dev.<nexus>.misc.devlog sysctl.

Sponsored by:	Chelsio Communications

git-svn-id: svn://svn.freebsd.org/base/stable/10@308318 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f