Add a crypto capability flag for accelerated software drivers.

Use this in GELI to print out a different message when accelerated
software such as AESNI is used vs plain software crypto.

While here, simplify the logic in GELI a bit for determing which type
of crypto driver was chosen the first time by examining the
capabilities of the matched driver after a single call to
crypto_newsession rather than making separate calls with different
flags.

Reviewed by: delphij
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D25126