Add warning for suspected slow system clocksource setting (#10636)

This PR does 2 main things:
1) Add warning for suspected slow system clocksource setting. This is Linux specific.
2) Add a `--check-system` argument to redis which runs all system checks and prints a report.

## System checks
Add a command line option `--check-system` which runs all known system checks and provides
a report to stdout of which systems checks have failed with details on how to reconfigure the
system for optimized redis performance.
The `--system-check` mode exists with an appropriate error code after running all the checks.

## Slow clocksource details
We check the system's clocksource performance by running `clock_gettime()` in a loop and then
checking how much time was spent in a system call (via `getrusage()`). If we spend more than
10% of the time in the kernel then we print a warning. I verified that using the slow clock sources:
`acpi_pm` (~90% in the kernel on my laptop) and `xen` (~30% in the kernel on an ec2 `m4.large`)
we get this warning.

The check runs 5 system ticks so we can detect time spent in kernel at 20% jumps (0%,20%,40%...).
Anything more accurate will require the test to run longer. Typically 5 ticks are 50ms. This means
running the test on startup will delay startup by 50ms. To avoid this we make sure the test is only
executed in the `--check-system` mode.

For a quick startup check, we specifically warn if the we see the system is using the `xen` clocksource
which we know has bad performance and isn't recommended (at least on ec2). In such a case the
user should manually rung redis with `--check-system` to force the slower clocksource test described
above.

## Other changes in the PR

* All the system checks are now implemented as functions in _syscheck.c_.
  They are implemented using a standard interface (see details in _syscheck.c_).
  To do this I moved the checking functions `linuxOvercommitMemoryValue()`,
  `THPIsEnabled()`, `linuxMadvFreeForkBugCheck()` out of _server.c_ and _latency.c_
  and into the new _syscheck.c_. When moving these functions I made sure they don't
  depend on other functionality provided in _server.c_ and made them use a standard
  "check functions" interface. Specifically:
  * I removed all logging out of `linuxMadvFreeForkBugCheck()`. In case there's some
    unexpected error during the check aborts as before, but without any logging.
    It returns an error code 0 meaning the check didn't not complete.
  * All these functions now return 1 on success, -1 on failure, 0 in case the check itself
    cannot be completed.
  * The `linuxMadvFreeForkBugCheck()` function now internally calls `exit()` and not
    `exitFromChild()` because the latter is only available in _server.c_ and I wanted to
    remove that dependency. This isn't an because we don't need to worry about the
    child process created by the test doing anything related to the rdb/aof files which
    is why `exitFromChild()` was created.

* This also fixes parsing of other /proc/\<pid\>/stat fields to correctly handle spaces
  in the process name and be more robust in general. Not that before this fix the rss
  info in `INFO memory` was corrupt in case of spaces in the process name. To
  recreate just rename `redis-server` to `redis server`, start it, and run `INFO memory`.