Implementing a new $(sys.uptime) variable (Mantis #1134)

The $(sys.uptime) variable contains the number of mintues the system
has been up and running. It uses various techniques depending on the OS
platform to find the system uptime:

* sysinfo() for Linux platforms
* kstat() for Solaris platforms
* pstat_getproc() for HP-UX
* sysctl() for BSD platforms

Any platform which doesn't have one of these methods available will default
to a stat(/proc/1) to get the create time of the init process, which falls
within a few seconds of the boot time of the system. (Even though Linux and
Solaris have /proc, this method is not used on those platforms even as a
fallback.)

If any of these approaches fail, the routine falls back to running the
"/usr/bin/uptime" or "/bin/uptime" command, and its output is parsed into
days, hours, and minutes with a PCRE regexp, and converted to a usable time
for the variable. Thanks to Ted Zlatanov for the suggestion: https://github.com/cfengine/core/pull/46#issuecomment-10066632

This popen() approach is unlikely to be used very frequently - probably only if
the /proc filesystem isn't mounted - but it's a decent backstop to give a
system the best possible chance to set $(sys.uptime) successfully.

Two other closely related variables are introduced: $(sys.systime) and
$(sys.sysday). The former is simply the result of the time() system call,
which is useful in manipulating timestamps in HP-UX TCB password databases,
for example, and the latter is that value divided by 86,400 - the number of
seconds in a day - expressed as an integer. This is, of course, useful in the
/etc/shadow file to indicate the password change date.

Three new documents are added in docs/reference/vars to describe the
$(sys.uptime), $(sys.systime), and $(sys.sysday) variables.
BSD <sys/param.h> include is required for <sys/sysctl.h> header checks

See also jail.h, sys/mount.h checks.
64-bit time_t snprintf; cf_popen needs pipes.h - for sys.uptime var

The cf_popen() and cf_pclose() calls now need to have pipes.h included,
because it was factored out in commit 5be64d09bc4c9c9659d53d2b5bf61286f768acbf.

The time_t type needs the proper printf format to support both 32- and 64-bit
platforms. On older platforms which don't have a C99-compliant snprintf, the
"pub/snprintf.c" replacement is used, so that a "%jd" format can be used even
on such platforms. Normally they would use the PRIdMAX definition from
inttypes.h. See commit f6ba42d5b2988be1f94ee4e55baa174fd7906362.
Improve autotest to fail also when cf-agent execution fails.
Fix a typo
Merge pull request #729 from sigurdteigen/master

Fix segfault resulting from bad ScopeDelete
Merge pull request #728 from tdlee/master

Fix a typo
Fix deadlock in cf-serverd, see redmine #2864.

Logging is removed from the signal handler until it is reimplemented as
reentrant (now it calls vasprintf() which calls malloc(), which is not
reentrant).

Also marked exit(0) in the agent for future removal for the same reasons.
Change select timeout to 1 second.

Previously the worst case for exiting was 10 seconds after the signal arrival.
Making libpromises the same version as the release
Merge pull request #730 from cduclos/master

Making libpromises use the same version as the release
fix for bug #2182. Added more params for the promises hashing algo
Merge branch 'bugwork/2182-cmds'
Close all databases as the last thing before exiting

...and keep mutexes locked to make sure they are not reopened by some
background thread.
Missing bug fix into ChangeLog
Output messages when starting and exiting

...since our signal handlers are silent now.
Typo fixes and updates
Merge pull request #732 from jimis/sighandling

Avoid deadlocks and error messages when exiting
Fix tarball generated by make dist.
Merge pull request #733 from maciejpatucha/master

Fix generation of tarball when running make dist