This commit implements generating /etc/security/limits.d/20-pw-defaults.conf and
/etc/security/limits.d/25-pw-rlimits.conf files. The numbering is arbitrary and
may very well warrant being in the reverse order, however `man 5 limits.conf`
does not appear to specify the parsing order or even say exactly how multiples
matches will resolve, so the value can be adjusted later, if required.

The actual limit values, the match rule and even whether each file is to be
installed can be changed via the build system before compilation. Likewise
the files can be modified or (re)moved during distro package building phase.

The 20-pw-defaults.conf should only be installed on legacy systems lacking both
a modern kernel and up to date systemd, because all it does is set the current
Linux default. Accordingly its not installed by default.

Signed-off-by: Niklāvs Koļesņikovs <89q1r14hd@relay.firefox.com>