Add a file descriptor in struct url for netrc

When using libfetch in an application that drops privileges when fetching
like pkg(8) then user complain because the application does not read anymore
${HOME}/.netrc. Now a caller can prepare a fd to the said file and manually
assign it to the structure.

It is also a first step to allow to capsicumize libfetch applications

Reviewed by: allanjude, des
Approved by: des
Differential Revision: https://reviews.freebsd.org/D9678