configure.in:  tweak libmodbus not found behavior

If libmodbus is not found:
  - Fail if --with-libmodbus specified (drop Debian-specific error)
  - Continue (with diagnostic) if not specified
configure.in:  don't use $TCLCONFIG dirname if $TCLCONFIG is empty
linuxcnc.in:  make race condition less likely by increasing sleep

There's a race condition in the linuxcncrsh tests that's been dealt
with before by sleeping a second before comparing test output to
expected output.

This occasionally bites my buildbot, so hack it more by increasing the
sleep to five seconds.

The following example output shows the last several lines of
'gcode-output' truncated; the file is complete upon later inspection:

shutdown
+ wait
+ exit 0
--- expected-gcode-output	2014-01-29 17:54:34.000000000 -0600
+++ gcode-output		2014-01-29 18:44:02.666945642 -0600
@@ -209,9 +209,3 @@
 tlo_z = -0.400000

 sequence number 30.000000
-X = 0.000000
-Y = 0.000000
-Z = 0.400000
-toolno = 99999.000000
-tlo_z = -0.400000
-
*** [...]/tests/toolchanger/toolno-pocket-differ/nonrandom: \
    FAIL: checkresult exited with 1
sample-configs in ${datadir}/linuxcnc; rename docsdir to docdir

The sample configs must be present for linuxcnc, or errors are
generated, since the files are used as defaults at run-time.

Debian explicitly forbids run-time requirements of files in
/usr/share/doc [1], and Fedora does in [2], where it is unstated that
all /usr/share/doc files are automatically labeled as %doc.

Fedora developers recommended putting these files into
/usr/share/linuxcnc.

In addition, rename 'docsdir' to 'docdir', per GNU coding standards
[3], and allow it to be set through './configure --docdir='.

In addition, replace hard-coded '/usr/local/etc' with $sysconfdir.

[1] https://www.debian.org/doc/debian-policy/ch-docs.html#s12.3
[2] https://fedoraproject.org/wiki/Packaging:Guidelines#Documentation
[3] http://www.gnu.org/prep/standards/html_node/Directory-Variables.html
module_helper/Submakefile:  create ../libexec in recipe

That directory is created by other recipes, so this recipe's omission
resulted in a race that occasionally caused build failures:

  DEBUG: /usr/bin/ld: cannot open output file \
    ../libexec/linuxcnc_module_helper: No such file or directory