systemd,manual: set LimitNOFILE=65536

A typical OS imposed limit is 1024 open FD, which can bee too low when
there are hundreds of concurrent voice calls.

In systemd service file, set a super high limit of 65536.

In osmo-mgw's user manual, add section 'Configure limits' describing
this in detail.

Related: OS#6256
Related: osmo-bsc I26c4058484b11ff1d035a919bf88824c3af14e71
Change-Id: I46512517bc3b5bb90cac7643e7ac73afba398d36
2 files changed