path: don't lower-cases drive letters

In general path functions don't change the case of a path. Making an
exception for windows drive letters violates the principle of least
surprise.

Changing the drive letter case has caused a lot of issues, including
joyent/node#7031, joyent/node#7806 and lots of bikeshedding about
whether uppercase is the right case or lowercase.

This effectively reverts joyent/node@a05f973

Reviewed-by: Alexis Campailla <alexis@janeasystems.com>
Reviewed-by: Julien Gilli <julien.gilli@joyent.com>