core: fix detection of dead processes

Commit 5ba6985b moves the UNIT_VTABLE(u)->sigchld_event before systemd
actually reaps the zombie. Which leads to service_load_pid_file accepting
zombie as a valid pid.

This fixes timeouts like:
[ 2746.602243] systemd[1]: chronyd.service stop-sigterm timed out. Killing.
[ 2836.852545] systemd[1]: chronyd.service still around after SIGKILL. Ignoring.
[ 2927.102187] systemd[1]: chronyd.service stop-final-sigterm timed out. Killing.
[ 3017.352560] systemd[1]: chronyd.service still around after final SIGKILL. Entering failed mode.
man: fix grammatical errors and other formatting issues

* standardize capitalization of STDIN, STDOUT, and STDERR
* reword some sentences for clarity
* reflow some very long lines to be shorter than ~80 characters
* add some missing <literal>, <constant>, <varname>, <option>, and <filename> tags
man: replace STDOUT with standard output, etc.

Actually 'STDOUT' is something that doesn't appear anywhere: in the
stdlib we have 'stdin', and there's only the constant STDOUT_FILENO,
so there's no reason to use capitals. When refering to code,
STDOUT/STDOUT/STDERR are replaced with stdin/stdout/stderr, and in
other places they are replaced with normal phrases like standard
output, etc.
man: use spaces instead of tabs

Several sections of the man pages included intermixed tabs and spaces;
this commit replaces all tabs with spaces.
util: fix mismatching function signature