usign-exec: improve usign -F output handling

usign-exec: improve usign -F output handling

While not likely to happen in pratice, nothing guarantees that read()
will retrieve more than 1 byte at a time. The easiest way to make this
code compliant is to wrap the file descriptor using fdopen().

While we're at it, also
- remove useless memset()
- check fingerprint for validity

The check is particularly relevant, as a usign bug [1] causing short
fingerprint outputs only went unnoticed for so long because the trailing
newline was considered one of the 16 characters ucert was expecting.

[1] https://patchwork.ozlabs.org/project/openwrt/patch/8ead1fd6a61117b54b4efd5111fe0d19e4eef9c5.1589642591.git.mschiffer@universe-factory.net/

Signed-off-by: Matthias Schiffer <mschiffer@universe-factory.net>