Add systemd socket activation for epmd.

Check for systemd libraries and headers in confgiure.
Add -systemd option enabling waiting for sockets from systemd.
Fix typo in common_test documentation for the -join_specs flag
Remove a superfluous echo in the emulator Makefile

It pollutes the output of silent rules.
Replace local mapl/2 (Erlang < 5.0) unique call by a LC
test_server: Allow init_per_ function to throw skip

Allow all init_per_[suite|group|testcase] to throw (or exit)
{skip, Reason} to explicitly ignore tests without failing.

It is easier to catch it in one place instead of writing a try
in each init_per_* to return {skip, Reason}.
Pass full strings to DTrace probes

Whenever string is passed as an argument to a DTrace probe, its length
should be properly computed. Until now in order to get length of the
input buffer size_of(char *) was used - which evalutes to 4 or 8
(depending on the architecture). To get a proper length,
size_of(DTRACE_CHARBUF_NAME(buffer_name)) should be used.
erts: Fix file driver to handle long paths on windows
erts: Use GetFullPathNameW to construct abs paths from relative ones
erts: Fix long windows paths for compressed files
erts: Fix file_SUITE:make_del_dir for long paths

Windows do not accept long UNC paths for current working directory.
fix file_SUITE:cur_dir_0 for long windows paths
erts: Ignore reduntant slashes in windows paths
erts: Revert file:set_cwd impl for windows

No need to even try as CWD can not bee a long path anyway.
erts: Deprecate halfword emulator
[snmp/agent] Fixed the loop_mib test cases
[snmp/agent] Fixed request id generation

When testing the agent, it is started and then used for a series
(v1, v2, v3, ...) of tests. But the individual test cases are
use one instance of the test manager. Because of this, the requerst
id cannot be a counter starting from 1 or any counter for that
matter. Instead every request id was generated using random.
But this is obiously not a guarantee fo uniqueness, it merely
makes it unlikely that the request ids will be reused.
To get around this problem (without having to rewrite the
entire agent test suite) a simple (global) counter server is
introduced. It is started at the start of the agent test suite
(init_per_suite) and stopped at the end (end_per_suite).
erts: Fix is_map/1 spec
erts: Document map guard functions

* erlang:is_map/1
* erlang:map_size/1
[snmp/manager] Verify success for app start and stop in test cases

When starting and stopping apps verify success. If failure
to start or stop app force failure of test case or init/fin
of test case. Also, be more verbose regarding stop results
during test case finish.
[snmp/agent] CLeanup - unused vars and use of deprecated funcs