Description of problem:
gssproxy.service fails to start when / is read-only.
It is obviously a problem when trying to recover from a file system issue.

$ touch /var/tmp/foo
touch: cannot touch '/var/tmp/foo': Read-only file system
$ sudo systemctl start gssproxy
Job for gssproxy.service failed because the control process exited with error code.
See "systemctl status gssproxy.service" and "journalctl -xe" for details.

Some debugging reveals that no socket is in use, but gssproxy fails to unlink /var/lib/gssproxy/default.sock.

# strace -f /usr/sbin/gssproxy -D
...
[pid   868] unlink("/var/lib/gssproxy/default.sock") = -1 EROFS (Read-only file system)
[pid   868] umask(0111)                 = 0177
[pid   868] socket(AF_UNIX, SOCK_STREAM, 0) = 7
[pid   868] bind(7, {sa_family=AF_UNIX, sun_path="/var/lib/gssproxy/default.sock"}, 110) = -1 EADDRINUSE (Address already in use)
...
[pid   868] writev(2, [{iov_base="gssproxy[868]: Failed to create "..., iov_len=72}, {iov_base="\n", iov_len=1}], 2gssproxy[868]: Failed to create Unix Socket! (98:Address already in use)

Please just move the socket to /run. That's what /run is for. (A static symlink
from /var/lib/gssproxy would be fine to keep backwards compat.)

Version-Release number of selected component (if applicable):
gssproxy-0.8.3-1.fc33.x86_64

How reproducible:
Deterministic

Steps to Reproduce:
1. Make /var/log read-only, either by not remounting the filesystems rw, or by using udev.blockdev-read-only with systemd-246+
2. Try to start sssd