Group session management routines together before first use.

• Rename cse*() to cse_*() to more closely match other local APIs in this file.

• Merge the old csecreate() into cryptodev_create_session() and rename the new function to cse_create().

Reviewed by: markj
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D27070