libpipeline: upgrade 1.5.6 -> 1.5.7

Changelog:
============
 * Add notes to libpipeline(3) of when functions were added.
 * Transferred Git repository to https://gitlab.com/libpipeline/libpipeline

.
 * Make `socketpair` tests used by `./configure --enable-socketpair-pipe`
   compatible with C23.

Signed-off-by: Wang Mingyu <wangmy@fujitsu.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>