Settings: Overhaul handling of rc-files.

Dunst now searches all possible locations of config files and applies
all settings from least important to most important file (see dunst(1)
for more details).

README.md: Added information about changed behaviour

dunst.{1,5}: Information about new settings behaviour and deduplication

dunst.1 explains in more detail how settings get found and applied.
SYSCONFDIR is now in sync with the compile time parameter.
dunst.5 got reduced a bit because of deduplication, so as to have only
one location for the respective relevant information and not have to
edit two or have them go out of sync.

utils.c: Minor fix, 'delimiter' in string_to_array() was never used.

settings_data.h: Drive-by fix

The default background of urgency_critical was same as foreground. Now
it is in sync with default dunstrc.

config.mk: Drive-by fix

According to the gcc man page '--std' does not exist but it seems to be
tolerated. Make that '-std' then.

log.h: Tweak some log messages

If the preprocessor allows it, prefix messages of levels 'critical',
'error', 'debug' with [<source path>:<function name>:<line number>].

Also some unified messages for logging fopen success/failure.

option_parser.c: Change handling of sections/entries.

Renamed `new_section()` to `get_or_create_section()` to be more
transparent about its inner workings. An already existing section
should not be fatal, just add/append entries to the *existing* one.
Also, directly calling `new_section()` seemed pointless, since
`add_entry()` would implicitly create sections as needed, or add to
existing ones anyway.

Not insisting on dying when there is a duplicate section in a config
file opens a lot of possibilities. Now one can simply concatenate config
files in arbitrary order and feed them to dunst on stdin, i.e.
    `cat $XDG_CONFIG_HOME/dunst/dunstrc \
         $XDG_CONFIG_HOME/dunst/dunstrc.d/*.conf \
     | dunst -conf -

Internally 'option_parser' now adds/appends *all* entries to their
respective sections, reusing existing ones. Empty sections will never be
created, at least not by `load_ini_file()`. The most important entry in
a section is the last one, because that will be the last to be committed
to settings. So there should never be a problem with duplicates: last
occurrence of a key dictates the final value.

settings.c: Fixed issues mentioned in PR dunst-project#917

`settings_init()` and `set_defaults()` are called only once now. Tweaked
settings_init().

`open_config_files()` now takes care of opening all config files,
including the one given as command line parameter.

doc: Update documentation

Added some additional information to address possible misunderstandings
about SYSCONFFILE and the runtime parameter -conf.