BUILD: systemd: fix usage of reserved name "sun" in the address field

BUILD: systemd: fix usage of reserved name "sun" in the address field

systemd.c doesn't build on Solaris / Illumos because it uses "sun" as
the field name in a structure, while "sun" is the name of the macro
used to detect Solaris:

  src/systemd.c: In function 'sd_notify':
  src/systemd.c:43:22: error: expected identifier or '(' before numeric constant
     struct sockaddr_un sun;
                        ^
  src/systemd.c:44:2: warning: no semicolon at end of struct or union
    } socket_addr = {
    ^

Admittedly, the OS could have instead defined "sun" to itself to avoid
this. Any other name will work, let's just use "ux" for the short form
of "unix".

The problem appeared in 3.0-dev with commit aa3632962f ("MEDIUM:
mworker: get rid of libsystemd"), though by then this file was only
built when USE_SYSTEMD was set, which was not the case for non-linux
platforms. However since 3.1-dev14 with commit 15845247db ("MEDIUM:
mworker: remove USE_SYSTEMD requirement for -Ws"), all platforms
now build this file.

No backport is needed even though it will not hurt to have it in 3.0
for completeness.