Fix issue with getenv failing

The call to `getenv` failed when `%` or `$` were used because of the
differences between Meterpreter handling and MSF handling.

Meterpreter effectively ignores (ie. strips out) the platform-specific
characters which are used for environment variables. In the `getenv`
call, MSF was invoking `getenvs` and getting a full hash of values, then
attempting to index into the hash using a string which may be "polluted"
with those platform-specific characters. This meant that there was a
discrepency between what was returned and what was used to index and
as a result, the value would come out as `nil`.

For example, calling `getenv('%FOO%')` would result in a hash with
`{'FOO'=>'bar'}`, so looking for '%FOO%' in this result would yield
nothing.

This commit changes this so that the name is ignored and the first
value is returned.
Land #2982, make getenv smarter