beinstall.sh: Use bectl instead of beadm by default

This patch also introduces an environment variable BE_UTILITY,
which can be used to specify the utility to use for managing
ZFS boot environments (which can be either bectl or beadm).

While here, fix some typos in the manual page and
remove beadm from section "SEE ALSO".

Reviewed by: bcr, kevans, rpokala
Approved by: will
Differential Revision: https://reviews.freebsd.org/D21111