jexec: Add -e parameter to customize the environment

Currently, to define a new environment variable or modify an existing
one, we need to use env(1), which may or may not be available inside
the jail, especially in OCI containers created with the scratch
layer (i.e., those containers that are only a single static binary,
plus configuration files and related stuff). With this option, we
can specify environment variables of arbitrary length for the
specified process running inside the jail.

Reviewed by: jamie@
Approved by: jamie@
Differential Revision: https://reviews.freebsd.org/D54660