wmmemload: Use sysctl to get memory usage in FreeBSD.

Previously, kvm_read was used to get the memory usage in FreeBSD.  However,
this has been deprecated in favor of sysctl.  This patch implements this
change.

Some work had previously been done in this direction, as could be seen in
the file src/mem_freebsd_sysctl.c.  Since this file is no longer needed, it
has been removed.

Additionally, some formatting has been done on the remaining older code for
a more consistent style.
wmmemload: Recognize GNU/kFreeBSD as a possible OS during configure.
wmmemload: Remove unused variable.

Avoid the following compiler warning:
dockapp.c: In function ‘dockapp_open_window’:
dockapp.c:51:16: warning: variable ‘stat’ set but not used [-Wunused-but-set-variable]
     Status     stat;
                ^
wmmemload: Be consistent in signedness of percentages.

Avoid multiple compiler warnings such as the following:
main.c: In function ‘update’:
main.c:157:4: warning: pointer targets in passing argument 1 of ‘mem_getusage’ differ in signedness [-Wpointer-sign]
    mem_getusage(&mem_usage, &swap_usage, &mem_opts);
    ^
In file included from main.c:29:0:
mem.h:16:6: note: expected ‘int *’ but argument is of type ‘unsigned int *’
 void mem_getusage(int *per_mem, int *per_swap, const struct mem_options *opts);
      ^
wmmemload: Add suggested parentheses.

Avoid the following compiler warning:
mem_linux.c: In function ‘mem_init’:
mem_linux.c:52:40: warning: suggest parentheses around ‘&&’ within ‘||’ [-Wparentheses]
    if (version == 2 && patchlevel >= 5 && sublevel >= 1 || \
                                        ^
wmmemload: Consistent formatting.

Format the source so as to minimize warnings from checkpatch.pl in the
Window Maker source tree.
wmmemload: Read memory correctly with Linux kernel version >= 3.14.

Beginning with Linux kernel version 3.14, a new MemAvailable line appears
in /proc/meminfo, which caused wmmemload to look for Cached and Buffers one
line off of their correct location.