library: add support for displaying LXC container name

This commit adds a lxc container name to every proc_t.
If a process is not running in a container, then a '-'
will be provided, making such a field always sortable.

Unlike other proc_t character pointers, lxc containers
will find many duplicate shared values. So rather than
strdup 'em (with a later free required upon reuse), we
try to keep track of those already seen and share that
address among all tasks running within each container.

We rely on the lines in the task's cgroup subdirectory
which may initially seem somewhat unsophisticated. But
the lxc library itself uses a similar approach when it
is called to list active containers. In that case, the
/proc/net/unix directory is parsed for the '/lxc' eye-
catcher, with potential complications from hashed path
and names that are too long (something we don't face).

[ too bad docker abandoned lxc - our commit won't do ]
[ anything for the users of those kind of containers ]

Reference(s):
https://bugs.launchpad.net/ubuntu/+source/lxc/+bug/1424253
https://bugs.launchpad.net/ubuntu/+source/procps/+bug/1424253

Signed-off-by: Jim Warner <james.warner@comcast.net>

proc/readproc.h

@@ -181,6 +181,8 @@ typedef struct proc_t {
         *sd_unit,       // n/a             systemd system unit id
         *sd_uunit;      // n/a             systemd user unit id
 #endif
+    const char
+        *lxcname;       // n/a             lxc container name
 } proc_t;
 
 // PROCTAB: data structure holding the persistent information readproc needs
@@ -292,6 +294,7 @@ extern proc_t * get_proc_stats(pid_t pid, proc_t *p);
 #define PROC_FILLOOM         0x0800 // fill in proc_t oom_score and oom_adj
 #define PROC_FILLNS          0x8000 // fill in proc_t namespace information
 #define PROC_FILLSYSTEMD    0x80000 // fill in proc_t systemd information
+#define PROC_FILL_LXC      0x800000 // fill in proc_t lxcname, if possible
 
 #define PROC_LOOSE_TASKS     0x2000 // treat threads as if they were processes