tcplxc: properly set the flags on bind mounts

Setting the mount flags on bind mounts requires 2 calls: 1 to perform
the mount, the second to remount the filesystem with the appropriate
flags. According to the man page, the bind flag is required in both
mounts:

    Note that behavior of the remount operation depends on the /etc/mtab
    file. The first command stores the 'bind' flag to the /etc/mtab
    file and the second command reads the flag from the file.  If you
    have a system without the /etc/mtab  file  or  if  you  explicitly
    define source and target for the remount command (then mount(8)
    does not read /etc/mtab), then you have to use bind flag (or option)
    for the remount command too. For example:

        mount --bind olddir newdir
        mount -o remount,ro,bind olddir newdir
tcplxc: allow creating system files

Mount /etc as a tmpfs filesystem. By default, create passwd and group
files. These can be overwritten by the user.

System files are created while running inside the read-only mount
namespace, so attempts to write the bind mounted system directories will
fail.
tcplxc: set default environment variables

Default environment variables can be overriden by the user.

Rename the proplist keyword from 'env' to 'environ', since it clashes
with the 'env' option to alcove_drv:start/1. These options should be
namespaced or put into another proplist to prevent this.