find: use sysconf(_SC_ARG_MAX) to determine the command-line size limit

The find utility uses a hardcoded value of 32 * 1024 as the limit of
the command-line length when calling 'find -exec ... {} +'. This results
in over 4 times more execve() calls than in coreutils' find.

This patch uses the limit defined in system headers.

Based on the patch by  Bartosz Golaszewski <bartekgola@gmail.com>.

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
libbb: use a wrapper around sysconf(_SC_CLK_TCK) to save a few bytes

function                                             old     new   delta
bb_sc_clk_tck                                          -      10     +10
timescmd                                             118     113      -5
print_route                                         1763    1758      -5
mpstat_main                                         1288    1283      -5
iostat_main                                         1947    1942      -5
INET_setroute                                        879     871      -8
------------------------------------------------------------------------------
(add/remove: 1/0 grow/shrink: 0/5 up/down: 10/-28)            Total: -18 bytes
Signed-off-by: Bartosz Golaszewski <bartekgola@gmail.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
unit-tests: implement the unit-testing framework

This set of patches adds a simple unit-testing framework to Busybox

unit-tests: add some helper macros for unit-test framework implementation
unit-tests: implement the unit-testing framework
unit-tests: add basic documentation on writing the unit test cases
unit-tests: modify the Makefile 'test' target to run unit-tests too
unit-tests: add two example test cases
unit-tests: modify the existing strrstr test code to use the unit-test framework

Signed-off-by: Bartosz Golaszewski <bartekgola@gmail.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
unlink: new applet

function                                             old     new   delta
unlink_main                                            -      45     +45
packed_usage                                       29667   29686     +19

Signed-off-by: Isaac Dunham <ibid.ag@gmail.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>