fileio: make parse_env_file() return number of parsed items

This commit introduces possibility to call parse_env_file_internal() and hand
over extra argument where we will accumulate how many items were successfully
parsed and pushed by callback. We make use of this in parse_env_file() and
return number of parsed items on success instead of always returning zero.

As a side-effect this commit should fix bug that locale settings in
/etc/locale.conf are not overriden by options passed via kernel command line.
localectl: print warning when there are options given on kernel cmdline
bus-proxyd: add some asserts

Both as documentation, and to make Coverity happy.

Fixes CID #1241495 and #1241496.
shared: path-util - try to make PATH_FORECH_PREFIX look less wrong

We replace the idiom "X && !(*foo = 0)" with "X && ((*foo = 0), true)".

This is not a functional change, but should hopefully make it less
likely that people and static analyzers believe there is a typo here
(i.e., to make it clear that the intention was not "X && *foo != 0").

Thanks to David Herrmann for the suggestion.
bus-proxy: drop one wrong assert()
readahead: wipe out readahead
delta: warn if diff failed

Found by Coverity. Fixes CID #1237541.
nspawn: check some more return values

Most of these failures would anyway get caught later on, but now the error messages are a bit more
specific.
journal-remote: initialize writer hashmap before use

https://bugs.freedesktop.org/show_bug.cgi?id=83682

[zj: move the initalization even earlier, before any sockets are
     looked at.]
journal-remote: fix counting of events written

After recent changes the number was always reported as 0, because
the accounting was done server_destroy(), called after the message was
already printed. But even before this change, the counts were wrong
because seqnum start at 0 only for newly created journal files, so when
appending to existing files, the calculated count was wrong anyway.

Also do some variable renaming for consistency and disable some low-level
debug messages.
journal: build fix when LZ4 is enabled but XZ is not
man/sd_event_new: tweaks
build-sys: add sd_session_get_desktop to Makefile-man.am

Fixup for c72d5456e2d.
man: add sd_event_add_signal(3)
man: add sd_event_add_child(3)
man: document sd_event_add_{defer,post,exit}
man: use <constant> markup for errno value

We were inconsistent, but marking them up as constants makes more
sense then as variables.
only build and install systemd-bus-proxyd if --enable-kdbus
build-sys: do not distribute make-man-rules.py

It was added to EXTRA_DIST in 3c3e5f4276a893791110b03984735654372aa33a,
but this script only makes sense for developers.
do not install factory/etc/pam.d if --disable-pam