build-sys: add a variable to collect .wants links

The idea is to make Makefile.am more declarative and avoid
repetitions. Redeclaring unit links as variables also makes
it easier to conditionally install only some of them.
build-sys: add variables to collect unit aliases
build-sys: link runlevel targets on install only if sysv compat is enabled
build-sys: add silent rule for m4 processing

Using custom prefixes makes the whole build process a bit more
readable.
build-sys: add silent rules for xslt processing
build-sys: add silent rules for gperf generation
service: really stop watchdog timer when stopping

For services without ExecStop= the state SERVICE_STOP is never entered. as
a result the watchdog timer is not stopped and the service is restarted (if
it is configuered to restart).
Stopping the watchdog timer for SERVICE_STOP_SIGTERM as well fixes this.
service: make sure the watchdog timer is not restarted while stopping

A watchdog notification may be handled after the watchdog timer was stopped
while stopping the service. As a result the timer is restarted and the
service may be restarted as well.
The watchdog timestamp is initially set during startup in
service_enter_start_post() and cleared when the timer is stopped. Therefore
it can be used as an indication if the timer should be reset.
core: Set source for mountinfo .mount units
man: systemd.exec - explicit Environment assignment

Hi all,

while working on another bug, I discovered the "strange" way systemd is
parsing Environment= in .service and thought it was worth documenting
(because I don't expect people to find this syntax by themselves unless
they read the parsing code ;)

Be more verbose about using space in Environment field and not
using value of other variables

Fixes https://bugzilla.redhat.com/show_bug.cgi?id=840260

[zj: expand and reformat the example a bit]