common: add pidfd related methods

Add functions to open pidfds, i.e. file descriptors representing
processes, for process ids and vice versa. Both functions work
an array of fds/pids, stop on error and return the number of
successfully handled items.

common/meson.build

@@ -5,12 +5,14 @@ common_sources = [
     'common-external.c',
     'common-helpers.c',
     'common-gpu.c',
+    'common-pidfds.c',
 ]
 
 daemon_common = static_library(
     'daemon-common',
     sources: common_sources,
     install: false,
+    include_directories: [config_h_dir]
 )
 
 link_daemon_common = declare_dependency(