Fixes exif corruption issues in jhead.

- Incorrect usercomment handling (backported from jhead 2.90)
- Doesn't handle (-1) sized SSHORT params, corrupts EXIF
- Doesn't convert floating point number strings to rational numbers, corrupts EXIF.
- Doesn't handle the separator ":" in addition to "/"

Fixes for the first two issues are provided by ANDREW COLLINS <w36194@motorola.com>.

Bug: 6775455

Change-Id: I5b93a2e61d47ccf0504f29ecde6703b358e97f63