[PATCH] percent-encoding for libfetch

Libfetch supports a username and password in a FTP or HTTP URL, but lacks
a method to specify '_at_' or ':' there, as they're used as URL component
separators.  I discovered this issue because I have an FTP server that
uses an email address as the username, and I can't use fetch(1) or
libfetch against it.

The attached patch adds decoding of percent-encoded usernames and
passwords, in order to use a URL like
ftp://foo%40example.com:password_at_host:port/file.bar .

Please review.  I plan to commit in the next few days.

-Ed