libfetch: Reject control characters in credentials

[des]

Summary

If a URL contains credentials, check that neither the user name nor the
password contain control characters.

MFC after: 1 week

[markj]

lib/libfetch/fetch.c

389

Shouldn't this be iscntrl()? isprint() will reject characters above 0x7f too, I'm not sure if that's desirable here.

[des]

use iscntrl() instead of !isprint()

[markj]

The commit log message should explain why.