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.

meson.build

@@ -128,9 +128,13 @@ with_examples = get_option('with-examples')
 with_util = get_option('with-util')
 
 # Provide a config.h
+pidfd_open = cc.has_function('pidfd_open', args: '-D_GNU_SOURCE')
+
 cdata = configuration_data()
 cdata.set_quoted('LIBEXECDIR', path_libexecdir)
 cdata.set_quoted('GAMEMODE_VERSION', meson.project_version())
+cdata.set10('HAVE_FN_PIDFD_OPEN', pidfd_open)
+
 config_h = configure_file(
     configuration: cdata,
     output: 'build-config.h',