procd: backport fixes from master branch

procd: backport fixes from master branch

The following commits have been cherry-picked into the lede-17.01
branch of procd, listed here in git-log-order ie. with head first:

89918c8 system: introduce new attribute board_name
(79bbe6d and 453116e on master branch)

8297c38 preinit: define _GNU_SOURCE
(e5b963a on master branch)

8fd57dd upgraded: cmake: Find and include uloop.h
(e5ff8ca on master branch)

6b0da20 hotplug: fix a memory leak in handle_button_complete()
(f367ec6 on master branch)

558ffb5 service/service_stopped(): fix a use-after-free
(796ba3b on master branch)

22f89e1 upgraded: define __GNU_SOURCE
(e7bb2c8 on master branch)

6e8ea8b rcS: add missing fcntl.h include
(992b796 on master branch)

cd5225d procd/rcS: Use /dev/null as stdin
(d42b21e on master branch)

5131bec procd: Log initscript output prefixed with script name
(1247db1 on master branch)

225b18d procd: Don't use syslog before its initialization
(8d720b2 on master branch)

889442c procd: Add missing \n in debug message
(2555474 on master branch)

2716228 procd: service gets deleted when its last instance is freed
(8f218f5 on master branch)

Signed-off-by: Daniel Golle <daniel@makrotopia.org>