Add os:unsetenv/1

New BIF os:unsetenv/1 which deletes an environment variable and
returns 'true'.

Does not change any old functionality.

Calls the libc function unsetenv(3) on UNIX and
SetEnvironmentVariableW(key, NULL) on Windows. The unicode support
is the same as for os:getenv and os:putenv.
Update primary bootstrap
erts: remove ?line macro in testcases
erts: Fix cerl -gdb

by replacing all newlines in $beam_args with space
[eldap, inets]: fix testcases when ssl not is present
Merge branch 'sverk/cerl-gdb-fix' into maint

* sverk/cerl-gdb-fix:
  erts: Fix cerl -gdb
Merge branch 'maint'
Merge branch 'mhssler/add-os-unsetenv/OTP-11446' into maint

* mhssler/add-os-unsetenv/OTP-11446:
  erts: remove ?line macro in testcases
  Update primary bootstrap
  Add os:unsetenv/1
Merge branch 'maint'

Conflicts:
	bootstrap/lib/kernel/ebin/os.beam
Merge branch 'fredrik/fix_tests_ssl_not_present/OTP-11474' into maint

* fredrik/fix_tests_ssl_not_present/OTP-11474:
  [eldap, inets]: fix testcases when ssl not is present
Merge branch 'maint'