The code concerning quoted-printable encoding (RFC 2045) and its
variant for MIME headers (RFC 2047) is currently spread in
mime/multipart and net/mail. It is also not exported.

This commit is the second step to fix that issue. It moves the
RFC 2047 encoding and decoding functions from net/mail to
internal/mime. The exported API is unchanged.

Updates #4943

Change-Id: I5f58aa58e74bbe4ec91b2e9b8c81921338053b00
Reviewed-on: https://go-review.googlesource.com/2101Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>