Fix a cluster of bugs in list EFI environment variables:

1. Size returned for variable name is in bytes, not CHAR16 (the UEFI standard is unclear on this, where it is clear on the size of the variable).
2. Dynamically allocate the buffers so we can grow them if someone defines a super-long variable name.

These two fixes allow me to examine all the variables in my BIOS and
also removes the repeated printing of variables.