iconv: Add UTF-7-IMAP variant in utf-7.c

iconv: Add UTF-7-IMAP variant in utf-7.c

UTF-7-IMAP differs from UTF-7 in the followings ways (see RFC 3501[1]
for reference) :

- The shift character is '&' instead of '+'
- There is no "optional direct characters" and the "direct characters"
  set is different
- There is no implicit shift back to US-ASCII from BASE64, all BASE64
  sequences MUST be terminated with '-'

[1]: https://datatracker.ietf.org/doc/html/rfc3501#section-5.1.3

Signed-off-by: Max Gautier <mg@max.gautier.name>
Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>