man: wording and grammar updates

This is a recurring submission and includes corrections to various
issue spotted. I guess I can just skip over reporting ubiquitous comma
placement fixes…
Fix for SIGSEGV in systemd-bootchart on short-living processes

The function svg_ps_bars() dereferencess NULL pointer in the line
	endtime = ps->last->sampledata->sampletime;
because of partially initialized ps_struct (ps->last == NULL).

If some process terminates between scaning /proc directory in the log_sample()
function and reading additional information from /proc/PID/... files,
the files couldn't be read, the loop will be continued and partially
initialized structure returned.
man: document the -b special boot option
logind: allow unprivileged session-device access

The session-device/control API was introduced for unprivileged device
access from within a session. Add the required dbus policy to the default
logind policies.

Note: logind validates that only root and the user of a session can
use the API. Furthermore, only a single API user gets access at a time.
Add sabridge for socket activation of traditional daemons