TT#182450 systemd: allow LimitMEMLOCK usage for secure memory usage by libgcrypt

Fixes:

| Warning: using insecure memory!

as triggered via libgcrypt, see
https://sources.debian.org/src/libgcrypt20/1.10.1-2/src/secmem.c/?#L283

Let's use 8388608 AKA 8MB as default, as present in systemd versions
251 and newer, see commit:

| commit 852b62507b22c0a986032a2c9fa9cc464a5b7bd2
| Author: Lennart Poettering <lennart@poettering.net>
| Date:   Thu Mar 10 13:22:57 2022 +0100
|
|     pid1,nspawn: raise default RLIMIT_MEMLOCK to 8M
|
|     This mirrors a similar check in Linux kernel 5.16
|     (9dcc38e2813e0cd3b195940c98b181ce6ede8f20) that raised the
|     RLIMIT_MEMLOCK to 8M.
|
|     This change does two things: raise the default limit for nspawn
|     containers (where we try to mimic closely what the kernel does), and
|     bump it when running on old kernels which still have the lower setting.
|
|     Fixes: #16300
|     See: https://lwn.net/Articles/876288/

Change-Id: I56f6d173d316386501ce8b13cc7a8ad6bea4ed26

debian/ngcp-rtpengine-daemon.service

@@ -112,7 +112,8 @@ DevicePolicy=closed
 IPAddressAllow=any
 
 # Maximum number of bytes of memory that may be locked into RAM
-LimitMEMLOCK=0
+# NOTE: required for libgcrypt's mlock handling
+LimitMEMLOCK=8388608
 
 # Restrict system calls that are allowed to be executed
 # NOTE: @system-service => reasonable set of system calls used by common system services