Add System V semaphores and shared memory metrics

What did you expect to see?

Same metrics as:

# ipcs -u

------ Messages Status --------
allocated queues = 0
used headers = 0
used space = 0 bytes

------ Shared Memory Status --------
segments allocated 1117
pages allocated 1535
pages resident  1535
pages swapped   0
Swap performance: 0 attempts	 0 successes

------ Semaphore Status --------
used arrays = 1006
allocated semaphores = 1006

i. e. semctl(IPC_INFO, ...):

       IPC_INFO (Linux-specific)
                 Return information about system-wide semaphore limits and parameters in the structure pointed to by arg.__buf.  This structure  is  of  type  seminfo,  defined  in
                 <sys/sem.h> if the _GNU_SOURCE feature test macro is defined:

                     struct  seminfo {
                         int semmap;  /* Number of entries in semaphore
                                         map; unused within kernel */
                         int semmni;  /* Maximum number of semaphore sets */
                         int semmns;  /* Maximum number of semaphores in all
                                         semaphore sets */
                         int semmnu;  /* System-wide maximum number of undo
                                         structures; unused within kernel */
                         int semmsl;  /* Maximum number of semaphores in a
                                         set */
                         int semopm;  /* Maximum number of operations for
                                         semop(2) */
                         int semume;  /* Maximum number of undo entries per
                                         process; unused within kernel */
                         int semusz;  /* Size of struct sem_undo */
                         int semvmx;  /* Maximum semaphore value */
                         int semaem;  /* Max. value that can be recorded for
                                         semaphore adjustment (SEM_UNDO) */
                     };

                 The semmsl, semmns, semopm, and semmni settings can be changed via /proc/sys/kernel/sem; see proc(5) for details.

We need this because Apache is consuming a lot of semaphores on some servers.

What did you see instead?

nothing

Start by reading the semctl IPC_INFO entry point and the /proc/sys/kernel/sem settings referenced in the issue, then compare their available values with ipcs -u. Determine how System V semaphore and shared-memory metrics should be exposed in node_exporter; done means the requested metrics are collected and match the corresponding system values.