util: make use of isempty() where appropriate
copy: teach copy_bytes() sendfile() support, and then replace sendfile_full() by it
copy: change error code when hitting copy limit to EFBIG

After all, this is about files, not arguments, hence EFBIG is more
appropriate than E2BIG
fileio: simplify write_string_file_atomic() by reusing write_string_stream()
util: simplify proc_cmdline() to reuse get_process_cmdline()

Also, make all parsing of the kernel cmdline non-fatal.
util: rework /proc/cmdline parser to use unquote_first_word()
timer: reenable TIMER_ACTIVE timers when restarted

A timer configured with OnActiveSec will start its associated unit again
if the timer is stopped, then started. However, if the timer unit is
restarted -- with "systemctl restart", say -- this does not occur.

This commit ensures that TIMER_ACTIVE timers are re-enabled whenever the
timer is started, even if that's within a restart job.
update TODO
util: file_is_priv_sticky() is used internally in util.c only nowadays, make it static
util: simplify normalize_env_assignment() a bit