uclient-fetch: add option to read POST data from file

uclient-fetch: add option to read POST data from file

Passing post data in command line is convenient but has limited size,
and may become tricky to correctly escape passed data especially in
scripts.
This patch add the option --post-file so the data to post can be read
from a file.
Tested on x86/64.

Acked-by: Jo-Philipp Wich <jo@mein.io>
Signed-off-by: Gioacchino Mazzurco <gio@eigenlab.org>
Signed-off-by: Daniel Golle <daniel@makrotopia.org>
(cherry picked from commit c66098611f16806a936a3a3f92113a16382fc4d9)